If you create a disk image backup of win8.1 & then restore it, the result will not boot. With earlier versions of Windows including version 8, EasyBCD could fix this sort of thing by adding the Vista/win7 boot-loader -- that does not work with the win8.1 Prev. either. Paragon Drive Copy does work.
What I *think* is happening is Microsoft must have changed something with the win8.1 boot loader, because earlier versions don't seem to work, though it may be more involved than that. At the end of a backup restoration, Paragon Backup & Recover sets the drive/partition to work with the PC/laptop, & restoring a win8.1 backup outside of win8.1, it does not add it to the boot menu of the OS you're working in as it usually does.
What that means is that if you install the win8.1 Prev. either use Windows built-in backup or test [or research 1st] whatever backup software you normally use, bearing in mind that if you restore a backup of a regular win8.1 install & it doesn't work, you might well have to reinstall win8.1.
Likewise it means that if the win8.1 Prev. boot-loader setup is incompatible with your PC/laptop, & it may be, you might need to restore a prior backup of your system drive(s), including the 1st disk track, putting things back the way they were.