Upgrade status generates a report that serves as a sort of checklist to help you determine whether or not your site is ready to be upgraded to Drupal 9. It packs a lot of useful information into a single report. It's worth taking the time to install it on a copy of your Drupal 8 site and seeing what it has to say.
Upgrading is the process of moving your site from a previous major version of Drupal to a newer version, for example from Drupal 7 to Drupal 9. This consists of upgrading the codebase to the appropriate version and then migrating the data from your old site into the new one. Drupal 8 core contains two modules to help facilitate this process: Migrate Drupal and Migrate Drupal UI.
Make sure you understand the difference between updating and upgrading. If you already have a Drupal 8 site and want to move to another minor version, see the Minor Version and Security Updates topic.
If you are on Drupal 8 and you want to upgrade to Drupal 9, a migration is not necessary. See Upgrade to Drupal 9
- Upgrade a site from Drupal 6 to Drupal 8
- Upgrade a site from Drupal 7 to Drupal 8
- Upgrade a site from Drupal 8 to Drupal 9
As of Drupal 8.5.x the core Migrate API is considered stable and no major backwards compatibility breaking changes are expected. If you're writing custom migrations, you're good to go.
Beware of any resources that were written before Drupal 8.1.x was released (April 19, 2016) as the system changed considerably in ways that invalidate many of the older articles.
Drupal check, and Drupal rector, are two useful command line tools you can use to help jump start the process of updating your Drupal 8 code to ensure it's compatible with Drupal 9. This post includes some notes about the process I went through while testing them out on some of the Drupalize.Me code base.