Here’s what I do …
- Make sure I have the install files with which I installed the old OS (+ Open GApps in my case).
- Use MyPhoneExplorer to “Multi-Sync” everything there is to sync … complete Internal Storage, contacts, messages, photos etc. … to my PC to be on the safe side with a data backup.
- Make a full system backup in TWRP and copy that to my PC.
Then I can wipe and format and test all I like, and if I want to have the old system back …
- Wipe everything in TWRP.
- (In case encryption is gone … disassemble new camera modules, install the old OS and immediately encrypt the phone again, encrypts very fast this way because there’s almost nothing in Internal Storage … reassemble new camera modules.)
- Restore the TWRP system backup.
- (In case the phone doesn’t want to boot afterwards, install the old OS again over the restored backup, this got it to boot reliably for me.)
- Boot the old OS and use MyPhoneExplorer to “Multi-Sync” all my data back to the phone
And voilà … back on the old system as if nothing happened 
.
Edit for a slight correction: My ringtones aren’t back after the restore.
The system backup restores the ringtone settings, but as my custom ringtone files don’t come back until Internal Storage gets restored, the OS cleverly reverts to default sounds.