This matter is usually covered by disclosures in update or upgrade documentation
for most software, and relies on the end user to make a choice based on their best
interests. Not on the latest software upgrade that could make other maker software
unworkable; especially if that other maker chose to not make new version upgrade.
When you upgrade any OS, you are like shifting gears and changing lanes; in haste
to get an engine overhaul and hopefully remember the brakes, could also need help.
With a partitioned hard drive or other bootable drive of suitable connectivity to use
if outside the main computer housing, you could try a newest supported OS while
not being fully committed to the follow-up. A new system always implies you have
to upgrade (get new) software. Some Mac users have their old Mac sitting aside &
fire it up for special tasks or routines that they don't want a newer lesser version of
a software that may or may not ever see an equal or better upgrade.
A production machine should never be upgraded without the responsible approach;
be that to read how other users of software or hardware have found a new thing to be
before taking a plunge into unknown waters. Some waters have stinger or biters there.
To revert to an older Mac OS X (from macOS Sierra) should be easy if you were prepared.
A Time Machine backup, and a backup clone that can run the system from external HDD.
Those should be minimal for a home user; the more backups of different types, the better.
For most recent Macs a good bit of information on using Time Machine & other stuff:
•Apple OS X & Time Machine Tips:
http://pondini.org/OSX/Home.html
That said, I have a few clones in externally enclosed self-powered hard drives sitting around.
And my newest Mac could have four systems installed -- if hard drives were partitioned.
Most older stuff is for my older Macs; some stuff is on DVD media, other on camera memory
cards that can be used with card reader for most anything. Be sure to have camera reformat
the card if you've used it otherwise. Same as USB Flash media. Or FireWire drives for clones
if you have an older reliably situated PowerPC G4/G5 pre-intel system on hand.
The 'tail that wags the dog' suggests several things that can happen; depending on priorities.
Was this idea from October 2016 a workable idea, if you tried it; or was it already past?
Re: Quickbook 2015 Not Working on Sierra OS
So perhaps you may see if you can backup your present upgrade, and then if you had extra
duplicate other backups from the earlier OS, restore to an earlier system using OS X Recovery
and perhaps the purchase history of an earlier system.
In any event...
Good luck & happy trails! 🙂
edited