The answer is easy, dead easy. Once you have upgraded one of your Windows 7 or 8.1 computers to Windows 10, on next boot, you will see something like this, a Windows Rollback option as one of the boot options.
I tried it first on my desktop test machine and it restored back to Windows 8.1 within 15 minutes with all my data, settings, and apps untouched. Very impressive.
But it didn’t go well with my Surface. When I selected Windows Rollback, it gave me a few more options, including USB restore media, Advanced options, etc., except for the one I was looking for, Windows 8.1. However, I did find another way to accomplish the same. If you encountered the same, try this:
Go to Start Menu → Settings → Update & Recovery → Recovery, and click Get started button under Go back to a previous version of Windows.
And again, the rollback process did an extremely well job reverting my Surface back to Windows 8.1, with all my data, settings, and apps untouched. Impressive again.
So, with so much easy to uninstall Windows 10 after the upgrade and rollback to what it was before, what’s really holding you back not upgrading your Windows 7 or 8.1 machines?