Make sure you have working backups, and test that they can be restored (e.g. to your local computer)
Before upgrading the production server, you can test if the migration works correctly and see how long it takes. For this take the backup from above and load it into a test database. Then run the latest 1.0 release with that database. If the test upgrade fails, open an issue.
On lemmy.ml the migrations take about 24 hours, during which the site will be inaccessible. Post an announcement about the planned upgrade time and use “Feature in Local” so that all users see it.