Downgrade your Mac is a very simple process. To begin you save time machine backup’s using your previous Mac OS. You have a great computer guy or you are great at computer DIY. My least favorite option I faced was wiping my MacBook completely clean.
I call Apple twice and downloaded Sierra to replace High Sierra. High Sierra on my end for past 4 months have not been compatible with my previous programs. Yesterday I just could not take it anymore, and I learned the hard way. Downgrading Mac OS’s or Mac OS X is impossible with have Time Machine backup on the Mac OS you want to move back to.
In my option, if the Mac OS is not 5 years old you should be able to downgrade at your desire. But that is not the case. You will be disappointed and get frustrated, but if you have prior Time Machine backup. Turn off your Mac, turn on your Mac, quickly hold SHIFT + OPTION + COMMAND + R. This should bring up a spinning globe for 40 minutes after the wait options should pop up. Time Machine reset should be the top option, plug in you External HD, click the dated backup and wait. If you don’t have a dated backup you are out of luck, per Apple reps states and my experience.
After my 35+ hour struggle, I finally found a great video that revealed my issue. This was the best video I found doing this process of downgrading Mac OS High Sierra back to OS Seirra. The key information I learned via youtube comments, was to delete and re-download the error 5 gig OS over again. Making a bootable USB 2.0 key worked perfectly as other past videos showed. [Watch video below to learn process]
I hope this cures your issue, now I re downloading and using migration assistant via time machine to put back some information for 1st time. -Marc