Your Time Machine backups are critical. They will give you the option to return your MacBook to its current state if needed.
Before commencing, back up all application data. I recommend exporting and/or converting data to all possible formats.
When you roll back your OS, the older Mail app won't be able to open your Mail database. If you currently use Microsoft Outlook, Mozilla Thunderbolt or some other third party app, this may not be an issue.
If you use the Photo app, export your photos as jpegs.
If you use iTunes, export your photos as mp3s. The older iTunes won't be able to open your iTunes database.
The older MacOS can't run the newer Mail, iTunes or Photos apps.
Now you may understand why I recommended installing your older MacOS on an external hard drive along with your QuickBooks programs. This ensures maximum uptime and functionality with a 1-time financial outlay and the minor inconvenience of having to use an external hard drive.
You would also be able to avoid needing to:
- Export data from all of your apps.
- Manually add photos, music, etc. added or create since that backup to the correct locations.
- Cry the blues over any missed data. It is all backed up so still recoverable.
If you use Stickies, its database might also be different.
iBooks is now a standalone app. It used to be integrated with iTunes. Do use it to store and synchronize pdf's.
If you currenty synchronize data (music, notes, contacts, pdf's, calendar, etc.) between your Mac and an iPhone, iPod Touch and/or an iPad using iTunes, please ignore all of the above. If you synchronize between iOS devices and your Mac using iCloud, if you have upgraded to iCloud Drive, you probably don't even want to roll back your primary OS.
If you install the older MacOS on an external hard drive and everything works perfectly after some tweaking, you can do a Time Machine backup of the external hard drive on a 3rd external hard drive (probably excluding the Mac's internal drive), do a final Time Machine backup on your original Time Machine backup then clone the external drive to your internal drive.
Howard