Similar situation here. I had Windows 7 Ultimate 64-bit and I upgraded first to Windows 8 Pro 64-bit and then to Windows 8.1 Pro 64-bit to have latest version. I want to change the serial to add Media Center. When I go to "Add Features to Windows 8.1", I get the error message that "you cannot add features to this version of Windows". From what I have found in Google, this is a bug and Microsoft has released a patch for Windows 8. But this patch doesn't work in 8.1 Do you know if there is a version for 8.1 or any other workaround? A Registry tweak maybe? Reinstalling Windows is out of the question as I have many applications and data and I don't like to spend Christmas reinstalling everything from scratch!
Thank you in advance.