I ran into issues last December when I upgraded a motherboard and CPU. It was with a retail Windows 8 key that I had used to upgrade to Windows 10 long ago under my MSA. I actually had been using the key under 8.1 for MCE use, but with a new CPU, I figured it was time to go to Win10. I was only using the key on one machine, same storage and case, etc—something I’ve done before. Still, Windows 10 wouldn’t activate, with cryptic error codes that basically said the key is not valid. I was signed into my MS account, which I thought was supposed to help with these sort of changes. I also tried an old Windows 7 key that had also been upgraded to 10, but same error.
Nothing I tried worked, so I called support. It took a few calls to India, and eventually MS remoted in to my PC, asked for visual proof of purchase of my new hardware, proof of purchase of my Windows 8 key (glad I kept the email), and then they gave me a new key. It was a really weird experience, to be honest. Much harder than it needed to be, IMO. They also told me that my old key for 8 will no longer work. I also have an iMac, and I must say, it’s nice to be able to install/reinstall macOS without these concerns. I’m still not sure why it all fell apart. Maybe I did too many hardware changes, but I thought that was not a problem with a retail key.