[dnanian]

In general, you should format your backup as APFS. Check it before upgrading. And realize that, of course, your backup is going to be "as of the time you back up" - if you downgrade, you're going to downgrade everything.

Other than that, though, there's no real trick here... the challenge is when you go from an OS that converts you to APFS to one that doesn't understand it (eg 10.14 -> 10.12)...