Here's (once again) my lecture on how to manage Apple software updates of any kind (iTunes, iWorks, macOS etc) for all those ringing your hands about the changes in iTunes 12.7 and being 'caught unawares'. First lesson, why do people update on their working, everyday systems, with working data? macOS is one of the EASIEST operating systems to clone and have multiple boot copies of.
Hard drive space is SO CHEAP; get yourself a couple terabyte external drive, and put multiple copies of bootable macOS on it. You don't need to go through the install process one by one, just clone your existing everyday partition, leaving out anything you don't want (data, certain apps, etc). I have one external drive with numurous bootable partitions, and not all the same version of macOS either. At least one each of High Sierra, Sierra, El Cap, Yosemite, or more. That's just one drive, I have other drives with boot partitions, going all the way back to Snow Leopard if I ever need it.. To clone just the OS portion only takes about 20GB partitions.
The point is you can boot into these non-everyday working partitions to test software and test upgrades. If you find an upgrade doesn't suit your taste, no harm done, reclone the partition and start fresh, or in THIS CASE, where people want to hold on to pre itunes 12.7 so they can manage their iOS apps from iTunes, great, keep a separate partition with 12.6 on it. Use that partition to manage iOS apps, use the 12.7 running on another partition to manage everything else to take advantage of the newer features and cleaner UI of 12.7. Apple at least now allows you to manage iOS devices from multiple copies of iTunes, as long as you confirm that it can be 'Trusted'. In the old days, attaching a device to another copy of iTunes required removing the existing data. In fact, to make content management easier for me, I use one copy of iTunes just for eBooks and audiobooks, and another copy on a different boot partition to manage music.
And this doesn't take a lot of work folks to set this up except for some initial time, I hear people already saying it's too hard to have multiple partitions, NO it isn't. One thing Apple did get right from day one is make macOS extremely flexible in regards to installing, and allowing Macs to boot from just about anything, and the same Mac to be able to boot from more than one partition. Windows users can't do that (without 3rd party tools). Windows users would KILL to be able to boot a PC from user selectable multiple Windows partitions, different physical drives, yes, but multiple partitions on the same drive, not so easy without hacks.
Now A**zon and N**egg have sales all the time on external portable drives. This morning, I see a 1TB USB 3.0 drive for under $50 on N**egg. CAn't afford $50? If you have space on your existing internal drive, set aside some space for another partition on that. Disk Utility EASILY allows you to shrink your existing partition and add another. Again, 20GB is plenty for a working copy of macOS, with no user data, plus whatever app you are wanting to test or update. SuperDuper cloning software is free, or my favorites CarbonCopyCloner and ChronoSync are very affordable. If you have a boot partition already of Yosemite or earlier, Apple's Disk Utility will also clone. Or for the brave, you can still clone using Terminal commands from any macOS. Numerous free Unix utils can also clone mac partitions (Gparted, PartitionMagic are a few), and yes, they work on a Mac because you boot from a flash key.
Last but not least, you can also use virtual machines to test software and upgrades before committing to it. If you already have VMWare Fusion or Parallels, test your upgrades inside a virtual machine. VirtualBox is free.
The people that hang out here at MacUpdate tend to be more technically advanced than your typical 'Uncle Henry and Aunt Em' users, so what I am describing here should be well within the technical abilities of many of MacUpdate users. With hard drives so inexpensive, and free software solutions available, cost should not be a barrier to implementing some of these processes. I test macOS updates the same way, never first time on my working everyday partition, with my working data; always on a clone first.