OK I understand now, your original Win 8.0 was an OEM version. Can you restore an image of your original Win 8.0 OS, do the 8.1 upgrade and then install a Pro 8.1 upgrade with your new key? If you keep an image of your current OS you can always go back to where you are now if that doesn't work. Can't think of another way around this - can your laptop support help?
I actually tried to do this but the version of Win 8 that came with the Sony laptop was from Nov 2012. There were over 190 updates and the updates failed after numerous tries. I could never get to the point of installing 8.1 since it required all windows updates to be applied? I restored my working copy of Win 8 Pro with MC and I'm still looking for a work around to get passed the UEFI problem. Any other suggestions? I really don't want to start from scratch if I don't have to since I have a large number of applications installed and it would take days "literally" to reinstall everything!!! Very frustrating.