I thought this idea would work but it didn't

Tim, welcome to these public User Forums.

Your idea should work in principle but there are various factors that could come into play that would give the issues you describe!

The first factor is the BIOS boot mode used by Windows 8.1 at the time of the backup image versus the BIOS boot mode now used by Windows 10.

You would need to look carefully at the contents of your backup images to see what system partitions are present in them?

If you see a EFI System partition then you have a UEFI BIOS boot system.

If you see a Microsoft System Reserved partition (with no EFI partition) then you have a Legacy BIOS boot system.

Any image restore has to be performed using the correct BIOS boot mode!

KB 59877: Acronis True Image: how to distinguish between UEFI and Legacy BIOS boot modes of Acronis Bootable Media

All I can tell you I guess is that when I made the Windows 8.1 images I did not make any changes to the default choices and boxes that Acronis checked for me, so I backed up the entire C:\ drive.

I believe I have a UEFI boot system. When I restart my computer and keep an eye on the bottom of the screen, I can see it say something like entering UEFI boot...

I don't think on my computer I can even disable UEFI because I don't see anything in the BIOS that says UEFI enable or disable.

So I assume that UEFI is always on.

Tim, one option you may want to consider for your scenario here, assuming that your PC will support this, is to use Hyper-V and restore your Win 8.1 backup image to a Hyper-V Virtual Machine rather than be changing between Win 10 and 8.1 in the way you describe!

I used to have dual-boot systems a few years back but after investing in a new computer 2-3 years ago, I switched to using VM's instead of using dual-boot.

These days I have a number of Win 10 and 11 plus a Ubuntu Hyper-V VM's along with a selection of VMware VM's with Win 10, 7, XP, 2000, ME, 98 & 95 machines, all of which can run on my main Win 11 desktop environment when needed.  The VMware VM's are mainly for historic interest rather than any real other use, but the Hyper-V VM's are where I do testing with different Acronis versions plus where I tested Win 11 before biting the bullet and upgrading my main work machine recently.

Hyper-V uses Gen 1 for Legacy systems and Gen 2 for UEFI, so make sure you select the correct VM gen type when setting one up if you decide to give this a try.

See my YouTube channel for some videos I have created using VM's that may be of interest.

No I don't think I want to get involved with a Virtual Machine after what I have read about them. If I decide to swap back and forth between operating systems, I'm going to have Windows 8.1 on a completely separate drive. I think it's better to keep Windows 8.1 and Windows 10 completely separated.

Tim, the decision will always be yours to make regarding using VM's.  They work well for me for what I use them for, but having separate drives for each of the 2 Windows OS is definitely better than having to keep restoring a backup image to switch between them.

If you can fit quick removal drive storage bays that can also make the swap much easier.