Serial number of windows 8.1 pro - Microsoft Community

Summary: The Win 8 to Win 8.1 upgrade is only free if you have a genuine Win8 system. The Win8 and Win8.1 product keys are not interchangeable. For your particular situation, you will need to reinstall Windows 8 using your Windows 8 product key and then upgrade to Windows 8.1 using one of the 2 methods in a and b. Regards