After updating to PMS 1.40+, you cannot directly roll back to a version less than 1.31.2.
Also, the database changes may take time - minutes to hours.
If you see the “503 maintenance” message, do not panic. Wait for Plex to finish the database migration. Check back every 30 minutes or so. The message will not auto-update. You’ll need to refresh the browser window or close/open a new window.