I built a new computer over the weekend and attempted to activate it with an unused Windows 8.1 pro product key. I have several that I’ve stockpiled over the years, and to date, I haven’t had a problem using any of them on Windows 10. Just out of curiosity, I installed and successfully activated an install of 8.1 with that key to confirm that it worked automatically, and it did.
The only difference between this and previous installs I’ve done is that in this case I was installing and activating 1703. I followed my normal process: installing with the default Pro key and then changing the product key to my desired key. It didn’t give me any reason why the key couldn’t be used.
One final curiosity was the Activation Troubleshooter: When I ran it, it stated that it found a valid Windows 10 S license for my PC. Being that my build was definitely not an OEM system that should have come with 10 S, I found it odd that the troubleshooter insisted that the appropriate license for the system was a SKU that isn’t available for purchase.