@AMP
I have my music system and Roon stored in boxes at the moment due to relocation. I briefly connected my ROCK server to perform the 2.0 upgrade +1.8 legacy downgrade. That went well.
When I start using my system again (in two-three months) can I use a database backup from June 2022 (1.8 ) to restore on 1.8 legacy?
If/when I decide to go from 1.8 legacy to 2.x, can I use the same backup from June 2022 (if needed)?
Thanks 
I’d recommend creating a newer backup, but you most likely won’t need to restore anything to change between these versions. They seem to be usable in either direction so to speak.
Problem is I disabled and disconnected my file library before I shut it all down for packing. It may all be there when I restart everything, but 1.8 legacy is now installed so I may have to restore from a backup. Moving from 1.8 legacy to 2.x should work, but one never knows…
From this statement it sounds like you migrated your core to 1.8 Legacy prior to packing it up. If that’s the case then you’re all set. There’s nothing more you need to do as you’re already on 1.8 Legacy.
This announcement was just to warn those who haven’t yet made the migration, but think that they might want to do so.
Problem with the transition from version 2.0 to version 1.8. I log out of my account, disable or delete version 2.0, then install version 1.8. I restore the backup and log in to my account. But the program writes about an error related to the fact that earlier a later version 2.0 was installed on this MacBook Pro 2012 and ask for technical support. I used uninstaller programs.
Is the backup you have from a 1.8 version. If not your trying to load in a database form 2.0 which is incompatible with 1.8 they gave a small window when they were backwards compatible but that went some time ago after a few new versions. If you don’t have a copy dating back to when you had 1.8 then you will need a fresh database and will loose all your edits and favourites.
2.0 database?
If you have been using a v 2.0 of Roon you cannot go backwards to 1.8 and maintain your database.
Database compatibility between Roon 2.0 and 1.8 Legacy ended with Roon 2.0 Build 1148 in November 2022:
If you have only a backup of a current 2.0 database, you cannot restore the backup with 1.8 Legacy. So you will need to start fresh with 1.8.
If you have kept an older backup, made before this 2.0 build 1148, then you probably can restore that
My database is 1.8
Maybe it helps if you delete the folder /Users/<username>/Library/Roon that was created by 2.0 before you install 1.8?