- Install the previous stable version of Moodle (latest stable release on the previous branch).
- Set the debugging level to DEVELOPER. Make sure you can see errors (display them on screen or monitor the error log).
- Set it up with some courses, users and content. Using the 'Make test course' feature in the Site administration is a quick way to achieve this.
- Upgrade the code to this current version of Moodle you are testing.
- Visit /admin and go through the upgrade process.
- On the page that lists the plugins to be upgraded, confirm that there are no "Missing from disk" plugins.
- Verify that the upgrade process completes without any errors.
- Repeat above steps after defining `$CFG->upgradekey` in the config.php. Make sure that when you attempt to upgrade the site via the web UI, the user must provide the valid upgradekey value. The non-admin user should have no access to any information regarding the site and/or plugins versions.