Best procedure for downgrading from Mojave to High Sierra
 

I have a MacBook Air (2017) that came with High Sierra installed. I would like to upgrade to Mojave (since it's the last OS that will run 32-bit apps) but would also like to be able to safely revert to High Sierra if I find something amiss with running Mojave.

I have read the thread "Downgrading from Mojave to High Sierra ... lessons learned" and would like to 'learn' what's relevant from it and also to not create my own sad lessons. So, before I begin, is there SuperDuper documentation I can study, and (also) are there specific states of my source and backup volumes that I should tend to before I begin?

My MBA has an SDD (2 partitions), with High Sierra installed on the 300gb partition, formatted APFS. (The other SDD partition is not bootable.) The backup is a SuperDuper-created copy (not image) to one 300gb partition, formatted Journaled HFS+, on an external 8gb USB drive.

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)...