I'm having major 'disagreements' with Sierra, compatibility issues with software etc, & have decided to revert to El Capitan.
I have done this many times in the past, tested an OS then gone back to the earlier version. I make a clone of the old drive, then update; clone back after a few days & all is well. Most I lose is a few non-essential emails which doesn't bother me.
This time, I've left it too long to do that. My clone is 2 weeks old, my POP email will only sync back one day. I have no real trace of what else may have changed in that period, but let's say with the best of intentions I'll miss something that I later discover I should have made a safety of.
I have Time Machine, up to date, but of course it's now syncing Sierra.
I also have Backblaze.
So ultimately I should have the resources to go back & find anything I may have missed.
My worry is if I use a clean install & migrate from Time Machine, some old authorisations on the drive will be lost. Some cannot be retrieved if they break because the servers that authorised them no longer exist. [Old pro audio plugins etc, still working but no longer supported.] These survive a clone, but I don't know if they'd survive a migrate. [I didn't migrate from my old 3,1 to this 5,1, I just moved the boot drive from the old to the new machine.]
I have all old installers in my Purchase History, so obtaining El Capitan is not an issue.
I'm on a Mac Pro 5,1 so dropping in a spare drive is no problem, if that would help.
I've never tested whether this machine will boot from USB, my older 3,1 never would - I shall test that & update the question.
Edit I've tested, it will successfully boot from an El Capitan installer USB. I've also made a clone of the boot drive as it is right now, belt & braces.
I see my options as...
1. Restore from the clone & try to find anything I missed.
2. Nuke & pave, then migrate Time Machine
Is there a 3rd option?
Some method to restore to my current backup, but sitting on El Capitan rather than Sierra?
Well, option 2 just went away. The installer simply refuses to migrate from a newer OS to an older one, whether from a mounted or remote drive, or Time Machine.
Restore Mavericks from Time Machine may help future Googlers too.