My current Drupal version is 8.9.3, running on PHP 7.3.20, and updated from Drupal 8.9.2 (fresh install) using compose update.

I tried to upgrade to Drupal 9 used this https://www.drupal.org/docs/upgrading-drupal/upgrading-from-drupal-8-to-drupal-9-or-higher as guide. So in step 2, I ran composer require "drupal/core:9.0.0 as 8.9.3" --no-update && composer update instead of composer require drupal/core-recommended:^9.0.0 drupal/core-composer-scaffold:^9.0.0 drupal/core-project-message:^9.0.0 --update-with-dependencies --no-update, but then Composer throws a conflict report regarding doctrine/reflection which conflicts with doctrine/common.

  Problem 1     - Conclusion: remove drupal/core-recommended 8.9.3     - drupal/core-recommended 8.8.x-dev requires drupal/core 8.8.x-dev -> satisfiable by drupal/core[8.8.x-dev].     - drupal/core-recommended 8.9.x-dev requires drupal/core 8.9.x-dev -> satisfiable by drupal/core[8.9.x-dev].     - Can only install one of: drupal/core[9.0.0, 8.8.x-dev].     - Can only install one of: drupal/core[9.0.0, 8.9.x-dev].     - drupal/core-recommended 8.9.2 requires drupal/core 8.9.2 -> satisfiable by drupal/core[8.9.2] but these conflict with your requirements or minimum-stability.     - drupal/core-recommended 8.9.1 requires drupal/core 8.9.1 -> satisfiable by drupal/core[8.9.1] but these conflict with your requirements or minimum-stability.     - drupal/core-recommended 8.9.0-rc1 requires drupal/core 8.9.0-rc1 -> satisfiable by drupal/core[8.9.0-rc1] but these conflict with your requirements or minimum-stability.     - drupal/core-recommended 8.9.0-beta3 requires drupal/core 8.9.0-beta3 -> satisfiable by drupal/core[8.9.0-beta3] but these conflict with your requirements or minimum-stability.     - drupal/core-recommended 8.9.0-beta2 requires drupal/core 8.9.0-beta2 -> satisfiable by drupal/core[8.9.0-beta2] but these conflict with your requirements or minimum-stability.     - drupal/core-recommended 8.9.0-beta1 requires drupal/core 8.9.0-beta1 -> satisfiable by drupal/core[8.9.0-beta1] but these conflict with your requirements or minimum-stability.     - drupal/core-recommended 8.9.0 requires drupal/core 8.9.0 -> satisfiable by drupal/core[8.9.0] but these conflict with your requirements or minimum-stability.     - drupal/core-recommended 8.8.8 requires drupal/core 8.8.8 -> satisfiable by drupal/core[8.8.8] but these conflict with your requirements or minimum-stability.     - drupal/core-recommended 8.8.7 requires drupal/core 8.8.7 -> satisfiable by drupal/core[8.8.7] but these conflict with your requirements or minimum-stability.     - drupal/core-recommended 8.8.6 requires drupal/core 8.8.6 -> satisfiable by drupal/core[8.8.6] but these conflict with your requirements or minimum-stability.     - drupal/core-recommended 8.8.5 requires drupal/core 8.8.5 -> satisfiable by drupal/core[8.8.5] but these conflict with your requirements or minimum-stability.     - drupal/core-recommended 8.8.4 requires drupal/core 8.8.4 -> satisfiable by drupal/core[8.8.4] but these conflict with your requirements or minimum-stability.     - drupal/core-recommended 8.8.3 requires drupal/core 8.8.3 -> satisfiable by drupal/core[8.8.3] but these conflict with your requirements or minimum-stability.     - drupal/core-recommended 8.8.2 requires drupal/core 8.8.2 -> satisfiable by drupal/core[8.8.2] but these conflict with your requirements or minimum-stability.     - drupal/core-recommended 8.8.1 requires drupal/core 8.8.1 -> satisfiable by drupal/core[8.8.1] but these conflict with your requirements or minimum-stability.     - drupal/core-recommended 8.8.0-rc1 requires drupal/core 8.8.0-rc1 -> satisfiable by drupal/core[8.8.0-rc1] but these conflict with your requirements or minimum-stability.     - drupal/core-recommended 8.8.0-beta1 requires drupal/core 8.8.0-beta1 -> satisfiable by drupal/core[8.8.0-beta1] but these conflict with your requirements or minimum-stability.     - drupal/core-recommended 8.8.0 requires drupal/core 8.8.0 -> satisfiable by drupal/core[8.8.0] but these conflict with your requirements or minimum-stability.     - Installation request for drupal/core 9.0.0 as 8.9.3 -> satisfiable by drupal/core[9.0.0].     - Installation request for drupal/core-recommended ^8.8 -> satisfiable by drupal/core-recommended[8.8.0, 8.8.0-beta1, 8.8.0-rc1, 8.8.1, 8.8.2, 8.8.3, 8.8.4, 8.8.5, 8.8.6, 8.8.7, 8.8.8, 8.8.x-dev, 8.9.0, 8.9.0-beta1, 8.9.0-beta2, 8.9.0-beta3, 8.9.0-rc1, 8.9.1, 8.9.2, 8.9.3, 8.9.x-dev].     - drupal/core 9.0.0 requires doctrine/reflection ^1.1 -> satisfiable by doctrine/reflection[1.1.x-dev, 1.2.0, 1.2.1, 1.2.x-dev, 1.3.x-dev, v1.1.0].     - doctrine/reflection 1.1.x-dev conflicts with doctrine/common[v2.7.3].     - doctrine/reflection 1.2.0 conflicts with doctrine/common[v2.7.3].     - doctrine/reflection 1.2.1 conflicts with doctrine/common[v2.7.3].     - doctrine/reflection 1.2.x-dev conflicts with doctrine/common[v2.7.3].     - doctrine/reflection 1.3.x-dev conflicts with doctrine/common[v2.7.3].     - doctrine/reflection v1.1.0 conflicts with doctrine/common[v2.7.3].     - drupal/core-recommended 8.9.3 requires doctrine/common v2.7.3 -> satisfiable by doctrine/common[v2.7.3].     - Conclusion: remove doctrine/common v2.7.3 

I don’t know what to do as I assumed it is a core modules conflict issue.

How do I fix this upgrade issue?

composer show shows the following output.

 asm89/stack-cors                     1.3.0           Cross-origin resource sharing library and stack middleware composer/installers                  v1.9.0          A multi-framework Composer library installer composer/semver                      1.5.1           Semver library that offers utilities, version constraint parsing and validation. container-interop/container-interop  1.2.0           Promoting the interoperability of container objects (DIC, SL, etc.) doctrine/annotations                 v1.4.0          Docblock Annotations Parser doctrine/cache                       v1.6.2          Caching library offering an object-oriented API for many cache backends doctrine/collections                 v1.4.0          Collections Abstraction library doctrine/common                      v2.7.3          Common Library for Doctrine projects doctrine/inflector                   v1.2.0          Common String Manipulations with regard to casing and singular/plural rules. doctrine/lexer                       1.0.2           PHP Doctrine Lexer parser library that can be used in Top-Down, Recursive Descent Parsers. drupal/admin_toolbar                 2.3.0           Provides a drop-down menu interface to the core Drupal Toolbar. drupal/allowed_formats               1.3.0           Limit which text formats are available for each field instance. drupal/amswap                        3.0.0           Allows the administration menu to be customised per user role. drupal/asset_injector                2.7.0           Adds CSS or JS to the page output based on configurable rules. drupal/auto_entitylabel              3.0.0-beta3     Allows hiding of entity label fields and automatic label creation. drupal/basic_watermark               1.0.0-alpha3    Watermark image style effect for Drupal. drupal/blazy                         2.1.0           Provides basic bLazy integration for lazy loading and multi-serving images. drupal/bootstrap_barrio              5.0.1           Bootstrap 4 base theme. drupal/bootstrap_library             1.13.0          Provides Bootstrap Integration. drupal/captcha                       1.1.0           The CAPTCHA module provides this feature to virtually any user facing web form on a Drupa... drupal/cdn                           3.5.0           Serves files (CSS, JS, images …) from a CDN. drupal/colorbox                      1.6.0           A light-weight, customizable lightbox plugin for jQuery. drupal/core                          8.9.3           Drupal is an open source content management platform powering millions of websites and ap... drupal/core-composer-scaffold        8.9.3           A flexible Composer project scaffold builder. drupal/core-project-message          8.9.3           Adds a message after Composer installation. drupal/core-recommended              8.9.3           Locked core dependencies; require this project INSTEAD OF drupal/core. drupal/ctools                        3.4.0           Provides a number of utility and helper APIs for Drupal developers and site builders. drupal/ds                            3.8.0           Extend the display options for every entity type. drupal/easy_breadcrumb               1.13.0          Adds configuration to the system breadcrumbs. drupal/entity_browser                2.5.0           Entity browsing and selecting component. drupal/entity_clone                  1.0.0-beta4     Add a clone action for all entities drupal/entity_reference_revisions    1.8.0           Entity Reference Revisions drupal/field_formatter_class         1.4.0           Provides custom HTML class settings for field formatters. drupal/field_group                   3.1.0           Provides the field_group module. drupal/field_permissions             1.0.0           The Field Permissions module allows site administrators to set field-level permissions to... drupal/file_download_link            1.0.0           Adds field formatter to render file field as configurable download link. drupal/file_mdm                      2.1.0           Provides a service to manage file metadata. drupal/file_mdm_exif                 2.1.0           Provides a file metadata plugin for EXIF image information. drupal/file_mdm_font                 2.1.0           Provides a file metadata plugin for TTF/OTF/WOFF font information. drupal/flexslider                    2.0.0-rc2       FlexSlider 2 integration for Drupal drupal/fontawesome                   2.17.0          The web's most popular icon set and toolkit. drupal/fontyourface                  3.2.0           Web font management tools. drupal/git_deploy                    2.3.0           Helps sites that are deployed directly from Git repositories. drupal/google_analytics              3.1.0           Allows your site to be tracked by Google Analytics by adding a Javascript tracking code t... drupal/image_effects                 3.0.0           Provides effects and operations for the Image API. drupal/imagefield_tokens             2.21.0          Provides new widget type for Image field to allow to use tokens for filling Alt and Title... drupal/libraries                     3.0.0-alpha1    Allows version-dependent and shared usage of external libraries in Drupal. drupal/login_security                1.5.0           Enable security options in the login flow of the site. drupal/mailsystem                    4.3.0           Mail System drupal/maxlength                     1.0.0-rc1       Limit the number of characters in textfields and textareas and shows the amount of charac... drupal/memcache                      2.1.0           High performance integration with memcache. drupal/menu_item_role_access         2.0.0           Provide role based access to menu items. drupal/metatag                       1.13.0          Manage meta tags for all entities. drupal/paragraphs                    1.12.0          Enables the creation of Paragraphs entities. drupal/paragraphs_collapsible        1.2.0           Improves the Paragraphs widget by applying CSS tweaks and collapsible feature drupal/pathauto                      1.8.0           Provides a mechanism for modules to automatically generate aliases for the content they m... drupal/pdf                           1.0.0           Display PDF file in Drupal without external readers and plugins. drupal/phpmailer                     3.0.0-beta1     Integrates the PHPMailer library for SMTP e-mail delivery. drupal/phpmailer_smtp                1.5.0           Uses the PHPMailer library to send emails via SMTP. drupal/recaptcha                     3.0.0           Protect your website from spam and abuse while letting real people pass through with ease. drupal/redirect                      1.6.0           Allows users to redirect from old URLs to new URLs. drupal/remove_http_headers           1.0.0-beta3     Removes configured HTTP Response headers. drupal/rename_admin_paths            2.0.0           The purpose of this module is to secure drupal backend by renaming the admin paths. drupal/shs                           1.0.0-alpha4    Creates a simple hierarchical select widget for taxonomy fields. drupal/slick                         2.2.0           Slick carousel, the last carousel you'll ever need. drupal/slick_views                   2.3.0           Provides Slick carousel integration with Views. Slick carousel, the last carousel you'll ... drupal/social_media                  1.8.0           Share current page to social media drupal/stickynav                     1.1.0           Module to make navigation on the website sticky when you scroll down the page. drupal/taxonomy_manager              1.0.0-beta2     Tool for administrating taxonomy terms. drupal/taxonomy_menu                 3.4.0           Creates menu items based on associations to taxonomy vocabularies drupal/token                         1.7.0           Provides a user interface for the Token API, some missing core tokens. drupal/token_filter                  1.2.0           This is a very simple module to make global token values available as an input filter. drupal/typed_data                    1.0.0-alpha5    Extends the core Typed Data API with new APIs and features. drupal/upgrade_status                2.9.0           Review current status of known Drupal 9 incompatibilities on the site. drupal/video_embed_field             2.4.0           A pluggable field type for storing videos from external video hosts such as Vimeo and You... drupal/views_fieldsets               3.x-dev eea8080 Adds fieldsets to Views. drupal/views_show_more               1.0.0           Provide a show more pager plugin for Views. drupal/xmlsitemap                    1.0.0           Creates XML sitemaps for the site easyrdf/easyrdf                      0.9.1           EasyRdf is a PHP library designed to make it easy to consume and produce RDF. egulias/email-validator              2.1.17          A library for validating emails against several RFCs google/recaptcha                     1.2.4           Client library for reCAPTCHA, a free service that protects websites from spam and abuse. guzzlehttp/guzzle                    6.5.4           Guzzle is a PHP HTTP client library guzzlehttp/promises                  v1.3.1          Guzzle promises library guzzlehttp/psr7                      1.6.1           PSR-7 message implementation that also provides common utility methods laminas/laminas-diactoros            1.8.7p2         PSR HTTP Message implementations laminas/laminas-escaper              2.6.1           Securely and safely escape HTML, HTML attributes, JavaScript, CSS, and URLs laminas/laminas-feed                 2.12.2          provides functionality for consuming RSS and Atom feeds laminas/laminas-servicemanager       3.4.1           Factory-Driven Dependency Injection Container laminas/laminas-stdlib               3.2.1           SPL extensions, array utilities, error handlers, and more laminas/laminas-text                 2.7.1           Create FIGlets and text-based tables laminas/laminas-zendframework-bridge 1.0.4           Alias legacy ZF class names to Laminas Project equivalents. lsolesen/pel                         0.9.8           PHP Exif Library. A library for reading and writing Exif headers in JPEG and TIFF images ... masterminds/html5                    2.3.0           An HTML5 parser and serializer. mathieuviossat/arraytotexttable      v1.0.8          Display arrays in terminal mglaman/phpstan-drupal               0.12.5          Drupal extension and rules for PHPStan nette/finder                         v2.5.2          🔍 Nette Finder: find files and directories with an intuitive API. nette/utils                          v3.1.2          🛠 Nette Utils: lightweight utilities for string & array manipulation, image handling, ... nikic/php-parser                     v4.7.0          A PHP parser written in PHP paragonie/random_compat              v9.99.99        PHP 5.x polyfill for random_bytes() and random_int() from PHP 7 pear/archive_tar                     1.4.9           Tar file management class with compression support (gzip, bzip2, lzma2) pear/console_getopt                  v1.4.3          More info available on: http://pear.php.net/package/Console_Getopt pear/pear-core-minimal               v1.10.10        Minimal set of PEAR core files to be used as composer dependency pear/pear_exception                  v1.0.1          The PEAR Exception base class. phenx/php-font-lib                   0.5.2           A library to read, parse, export and make subsets of different types of font files. phpmailer/phpmailer                  v6.1.7          PHPMailer is a full-featured email creation and transfer class for PHP phpstan/phpstan                      0.12.36         PHPStan - PHP Static Analysis Tool phpstan/phpstan-deprecation-rules    0.12.5          PHPStan rules for detecting usage of deprecated classes, methods, properties, constants a... psr/container                        1.0.0           Common Container Interface (PHP FIG PSR-11) psr/http-message                     1.0.1           Common interface for HTTP messages psr/log                              1.1.3           Common interface for logging libraries ralouphie/getallheaders              3.0.3           A polyfill for getallheaders. stack/builder                        v1.0.5          Builder for stack middlewares based on HttpKernelInterface. symfony-cmf/routing                  1.4.1           Extends the Symfony2 routing component for dynamic routes and chaining several routers symfony/class-loader                 v3.4.41         Symfony ClassLoader Component symfony/console                      v3.4.41         Symfony Console Component symfony/debug                        v3.4.41         Symfony Debug Component symfony/dependency-injection         v3.4.41         Symfony DependencyInjection Component symfony/event-dispatcher             v3.4.41         Symfony EventDispatcher Component symfony/http-foundation              v3.4.41         Symfony HttpFoundation Component symfony/http-kernel                  v3.4.41         Symfony HttpKernel Component symfony/polyfill-ctype               v1.17.0         Symfony polyfill for ctype functions symfony/polyfill-iconv               v1.17.0         Symfony polyfill for the Iconv extension symfony/polyfill-intl-idn            v1.17.0         Symfony polyfill for intl's idn_to_ascii and idn_to_utf8 functions symfony/polyfill-mbstring            v1.17.0         Symfony polyfill for the Mbstring extension symfony/polyfill-php56               v1.17.0         Symfony polyfill backporting some PHP 5.6+ features to lower PHP versions symfony/polyfill-php70               v1.17.0         Symfony polyfill backporting some PHP 7.0+ features to lower PHP versions symfony/polyfill-php72               v1.17.0         Symfony polyfill backporting some PHP 7.2+ features to lower PHP versions symfony/polyfill-util                v1.17.0         Symfony utilities for portability of PHP codes symfony/process                      v3.4.41         Symfony Process Component symfony/psr-http-message-bridge      v1.1.2          PSR HTTP message bridge symfony/routing                      v3.4.41         Symfony Routing Component symfony/serializer                   v3.4.41         Symfony Serializer Component symfony/translation                  v3.4.41         Symfony Translation Component symfony/validator                    v3.4.41         Symfony Validator Component symfony/yaml                         v3.4.41         Symfony Yaml Component twbs/bootstrap                       v4.5.2          The most popular front-end framework for developing responsive, mobile first projects on ... twig/twig                            v1.42.5         Twig, the flexible, fast, and secure template language for PHP typo3/phar-stream-wrapper            v3.1.4          Interceptors for PHP's native phar:// stream handling webflo/drupal-finder                 1.2.0           Helper class to locate a Drupal installation from a given path.