Backing up & treating Windows installs as sort of modular components works.
While it's generally recognized that a fresh install is better than an upgrade, that doesn't take into account things like years' worth of GOTD games & such. Likewise it doesn't take into account the fact I can't handle the extended concentration needed to migrate all my wife's games to win 8/8.1 from 7 & Vista -- get however many done & my brain starts to doze. Since the win 8/8.1 upgrade is mainly in the name of future-proofing rather than any sort of want, I also needed to preserve her win7 & Vista installations. This left me with two possibilities...
I could move 7 to a new partition, then upgrade it, or I could upgrade 7 right where it was, then move the upgraded Windows afterward. Either way is fairly trivial, but I went with the 2nd option because while moving 7 1st was less work, the odds are greater it would deactivate -- being mostly deaf I Truly Hate, Loath, Despise deactivation, spending the following week or so reminding myself that curses really don't work, & people at MS really aren't experiencing body parts suddenly dropping off. Anyway, here's how I pulled it off...
I first used Paragon to backup Vista & win7. I had already created a 3rd partition back when I replaced/installed that hard drive a year or so ago, just leaving it open or unallocated -- didn't need it so why bother defragging or error checking the added disk space. [If that hadn't been the case, then after that initial backup would have been the time to defrag 7's partition & shrink it, creating room for a 3rd. If I had needed/wanted to shrink Vista's partition I could have done that, deleted 7's partition, then restored the win7 backup right at the end of the Vista partition.] At any rate I upgraded win7, which did take quite a bit longer than a fresh win 8/8.1 install BTW, made sure all updates were added, ran the Disk Cleaner app [reclaiming ~18 GB], & backed up the new win8.1 install.
Then I booted into Vista, though I could have booted to a Paragon rescue disc [*.nix or WinPE], or used one of my generic WinPE-type discs/USB sticks [see reboot.pro] -- one reason I like multi-boot setups is this sort of thing is more convenient. In Vista I restored the win7 backup, then restored the win8.1 backup to the disc space I had left unallocated -- no need to bother creating a partition & formatting it when you're restoring a Paragon backup this way. The last step was running the free, super easy to use, portable, EasyBCD to set up the *standard* Vista, win7, & win8/8.1 BCD boot loader. [Win8/8.1 use a graphical interface rather than making you use the arrow keys on your keyboard to select what OS you want to boot into, but being able to use a mouse means graphics drivers are loaded, which means it's slow, & it means another reboot if you select anything but win8/8.1]
End of story, that's all there was too it, & the whole thing was pretty painless since I wasn't sitting there watching the screen waiting for whatever step to finish. My wife can use Vista & win7, booting into either exactly as before -- she just has one more option on the regular win7 boot menu for win8.1. I've still got drivers & such to hunt down, some software to update etc., decide what Start Menu app I'll use, but win8.1 works fine as-is if/when she chooses to use it.