Backup Migrate Downgrade

What is the recommended downgrade strategy without loosing data, downgrade by adb to previous version or copying the release to the phone and using the lineageos updater app?

I don't know of any way to downgrade without formatting / factory resetting and losing your data. I suggest the following:

1. Use either (or both) of the following to backup your user-installed apps and (some) data
   1. https://github.com/AndDiSa/android_backup_project/ - requires rooted debugging and a LInux machine, real or virtual. Will backup and restore all of your apps, and some data: See this post for guidance on how to use the tool and here for some info on what will and will not get restored
   2. https://github.com/mrrfv/open-android-backup will only backup apps, not their data, but that's the biggest hassle when setting up after a format. Doesn't need root or rooted debugging. Works on Linux, MacOS & Windows (though you may need an 'unstable' bundle on MacOS - see this issue
2. Use any in-app backup / export / sync functionality provided by the apps you use. e.g.
   1. K-9 has 'export settings,
   2. QKSMS has in-app backup functionality, or you can use SMS Import Export from F-droid for messages, call logs & contacts
   3. Lawnchair launcher has in-app backup
   4. DAVx5 / ICSx5 /NextCloud NOtes & Sync Client allow synching Contacts, Calendar, Notes, Tasks, and files / documents / photos

Then flash the new ROM using adb sideload without formatting your data partition: if you are lucky, the older version may work, but probably you will have to format, reboot and complete first time setup, then restore whatever you need from the backups, exports & syncs you made.