With Drupal 9 having reached its end of life (EOL) on November 1, it’s time to start planning for an upgrade.

For those already operating on Drupal 9, upgrading to Drupal 10 is not as daunting as earlier upgrades and promises to be easy, reflecting the software’s overall trend towards smaller, more incremental upgrades and faster iterations. According to Drupal, you should "completely update your Drupal 9 site to the most recent version of the modules and theme(s), before updating to Drupal 10." 

The deprecated code will be based on Drupal 9 moving into Drupal 10, but there are ways to search your system and update the specified code block.

The first thing to do is to install the  drupal/upgrade_status module in your project and enable it. composer require drupal/upgrade_status && drush en upgrade_status, if you have drush installed else go to admin > modules  and search for upgrade status and install it. After the module is enabled, go to Admin > Reports > Upgrade Status. This page should contain all the upgrades and code changes necessary before your site can be upgraded to Drupal 10.

Steps to upgrade Drupal 9 to Drupal 10

Migrating from Drupal 9 to Drupal 10 can be easy or can be difficult depending on the project you are involved in. Yes, because every site is different and may present its own unique challenges.

These are the following steps to migrate from Drupal 9 to Drupal 10.

1. Keeping your custom modules up to date with new standards and removing deprecated code will result in small changes before you have to do a major core upgrade. If the site is well maintained the changes are mostly with the core_version_requirement. You will need to update from core_version_requirement: ^9 to core_version_requirement: ^9 || ^10 . As soon as we are sure that our custom code is compatible with Drupal 10, we can move to check the contrib modules. And this is where things might get tricky

2. Upgrade contrib modules to versions which support Drupal 9 and 10. If the new version only supports D10, then you can add it to composer as an alternative version, for example: 'drupal/module_name': '^1 || ^2', and you can have version 1 still installed while on Drupal 9. Once you install Drupal 10, version 2 of the module will be installed by composer.

3. Uninstall obsolete modules and themes like Color, RDF, Seven etc. You can’t remove core modules or themes, but if you are not sure if these are depended upon, it can be moved from core to contrib especially classy and stable, which you can keep as contrib modules. You may need to set your Admin theme to Claro and re-export your config on this step. Use drush theme:uninstall theme(s) command (drush thun for short) to uninstall themes.

4. Remove orphaned permissions which are still assigned to user roles. Export your config before starting this step. The upgrade_status module will tell you which permissions need to go from which roles. Simply edit the user.role.[role].yml config ymls and remove the relevant lines from the permissions array. 

5. Upgrade Drupal core to latest 9.5, in order to upgrade to 10.

composer require drupal/core-recommended:^9.5 drupal/core-composer-scaffold:^9.5  --update-with-all-dependencies

Now let's look at the tricky part. Drupal 10 has some specific things we have to do to allow us to upgrade to drupal 10.

1. Drupal 10 runs with PHP 8.2 or later so make sure you have it to be able to upgrade. 

2. Not all modules are Drupal 10 ready so you will have to use drupal lenient.  You will have to get a patch for Drupal 9 only modules. Install mglaman/composer-drupal-lenient before attempting to upgrade. Otherwise these modules will create a dependency problem. After the installation, you need to declare the Drupal 9 modules which should be treated as Drupal 10 modules. For example, to allow drupal/token to be installed run:

composer require mglaman/composer-drupal-lenient
composer config --merge --json extra.drupal-lenient.allowed-list '["drupal/token"]'

3. If you have Drush installed make sure the version is ^11 || ^12, we use this because some module may not be version 12 compatible.

4. Drupal Console (drupal/console) is not Drupal 10 compatible and as such we have to remove it. 

5. Upgrade CKEditor 4 to CKEditor 5,if you have custom CKEditor 4 plugins which don’t have an upgrade path to CKEditor 5, then you can keep using CKEditor 4 but as a contrib module. You may have to use it although it is deprecated, to avoid errors when you deploy the Drupal 10 upgrade, when the config import tries to uninstall CKEditor.

composer remove drupal/console --no-update

6. If you use Entity Embed, you need to manually replace CKEditor Embed buttons with SVG icons as Drupal 10 is not using png.

7. Check custom code for core/jquery.once if it does exist update it to use core/once because jquery.once is removed from Drupal 10

8. If you encounter dependency issues, and composer just won’t let you upgrade to D10, but you are sure that all the dependencies are in order, then the quickest thing to do is to delete the composer.lock file(although its not a best practice), delete vendor folder then run composer install which will install from the composer.json file and create a new .lock file.

9. Note that you can ignore some false positives such as an empty custom profile, which phpstan can’t scan, and as such upgrade status would complain, the trick would be to create an empty .php file to get the issue fix

After all these are done and ready for upgrade, you can run this command

composer require 'drupal/core-recommended:^10' 'drupal/core-composer-scaffold:^10' 'drupal/core-project-message:^10' --no-update

If   drupal/core happens to be part of the composer.json file, remove it. This dependency is included in drupal/core-recommended and may cause problems. If you have drupal/core-dev installed, you can run this

composer require 'drupal/core-dev:^10' --dev --no-update

Now,let us test perform the update with the --dry-run option: this allows us to see if the update will runs smoothly or might encounter some errors.

composer update --dry-run

If you encounter any errors, walk through the process to resolve them. Resume the process when the errors are resolved, run the update with dependencies to update any transitive module.

composer update -W

Don’t forget to run drush updb and drush cex after the upgrade. This means you should run the upgrade on top of an installed and functioning D9 database.

If you have a custom theme that is  based on classy,seven or stable then don’t forget to install them as  contrib themes - composer require 'drupal/[module_name]'. You should uninstall and remove upgrade status after the upgrade process.

Pathauto has an issue with some config files so, if you have Pathauto installed then you need to update some configs by hand. Namely pathauto.pattern.[name] config files have had their selection_criteria plugins updated. For example node_type becomes entity_bundle:node. or  try the following command

drush eval 'include_once(DRUPAL_ROOT . "/modules/contrib/pathauto/pathauto.install"); pathauto_update_8108()'

In summary the whole update process for a site can be very difficult and ranges from project to project. It could have different modules installed, with some even locked to specific dev versions, as well as heavily patched old major versions of modules which needed to be upgraded. You walk through these steps and iterate through if need be to get your upgrade completed.