I built my own computer, and bought a copy of Windows 8 Pro about a year ago. Entered my product key and everything during setup, went smoothly. When Windows 8.1 came out I upgraded through the Windows Store app. I also wanted to try out Windows Media Center so, I'll be honest here, I looked up a product key for it (didn't buy) and it somehow worked, so I had Windows 8.1 Pro with Media Center. I just wanted to try it out, not keep it.
Unfortunately, the key was blocked, so Windows is now nagging me to activate every 3 hours or so. I want to return to my legit product key to stop this and return to Windows 8.1 Pro, but when I enter the key I bought (remember, it's a Windows 8 key, not 8.1), it says that the key can't be used to activate this edition of Windows. What should I do? I want to avoid clean re-installing Windows 8 if possible.