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1. Install the Drupal Upgrade Status module on your Drupal 9 site.

2. Review status /admin/reports/upgrade-status

3. Update Drupal core to 9.5 (or the latest version of 9 at the time of upgrade)

4. Permissions errors:

   1. Uninstall the module with broken permission

   2. Install it again

   3. Configure permissions (re-save permissions form)

   4. The error should be gone. More here

   5. If the website is using field_permissions module, access each field on every content type, and save the field again.

5. Update your version of PHP to at least 8.1.6

6. Make sure your site is running on it

7. Run composer why-not php 8.1 to see if any packages need updates (only works if PHP 8.1 is a composer dependency in your composer.json). I recommend including it as a dev dependency and removing it after an upgrade.Replace deprecated color extensions with the contrib modules if they are in use on your current website specifically: quick edit, RDF, Classy, Stable

8. Deprecated modules

   1. CKEditor has been replaced with CKEditor 5 in core. If you don’t use any modules that provide CKEDitor4 plugins that haven’t been updated yet, upgrade to CKEditor5. If you rely on older plugins, you can install the contrib module

   2. Color has been moved to contrib

   3. The Classy, Seven, Bartik, and Stable themes have moved to contrib so install those if you rely on those themes. See the step below for details on transitioning away from Classy.

   4. Unless your site really relies on them for some reason, it is probably best to remove: Quick Edit and RDF.

9. If your theme uses Classy as a base theme, determine if you’d like to keep Classy as a contrib dependency or if you’d like to transition off Classy. Read this change record to figure out the steps or download and install the contrib module. Keep in mind that Stable is deprecated as well and it’s a Classy dependency.

   1. Steps to transition off Classy

      1. Copy these templates from classy to custom theme templates directory:
         themes/custom/THEMENAME/templates/block/block--system-menu-block.html.twig
         themes/custom/THEMENAME/templates/block/block.html.twig
         themes/custom/THEMENAME/templates/form/checkboxes.html.twig
         themes/custom/THEMENAME/templates/form/container.html.twig
         themes/custom/THEMENAME/templates/form/fieldset.html.twig
         themes/custom/THEMENAME/templates/form/form-element.html.twig
         themes/custom/THEMENAME/templates/form/form.html.twig
         themes/custom/THEMENAME/templates/form/radios.html.twig
         themes/custom/THEMENAME/templates/form/select.html.twig
         themes/custom/THEMENAME/templates/form/textarea.html.twig
         themes/custom/THEMENAME/templates/layout/region.html.twig
         themes/custom/THEMENAME/templates/navigation/links.html.twig
         themes/custom/THEMENAME/templates/navigation/menu-local-action.html.twig
         themes/custom/THEMENAME/templates/navigation/menu-local-task.html.twig
         themes/custom/THEMENAME/templates/navigation/menu-local-tasks.html.twig
         themes/custom/THEMENAME/templates/navigation/menu.html.twig

      2. Set base_theme value to false (for D9 requirement): base_theme:false or remove base_theme property if on Drupal 10.

      3. Visually QA theme to check if there are other elements or markup form Classy that your styles depended on.

10. Remove uninstalled incompatible modules and themes

11. Update contrib modules to their compatible latest versions and test the functionality

12. Scan custom modules for deprecations and fix with Rector or manually

    1. Documentation to remove deprecated jQuery.once calls: https://www.drupal.org/node/3158256

    2. Documentation to remove deprecated jQuery.cookie calls: https://www.drupal.org/node/3104677

13. If the site is on Pantheon, update pantheon.upstream.yml or pantheon.yml with the correct version of PHP

14. Push your changes to the multidev, test the functionality, and ask PM and the client to test

15. Merge changes with Dev branch, and retest. Move them to test and test with the client

16. Schedule the deployment, make the backup, and deploy to live

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