I hope the following helps. I had Windows 8 preview and to upgrade I used the back up discs I created for Windows 7pro and ran those to reset my computer to Windows 7pro factory settings.
I then went to the Windows site at: http://windows.microsoft.com/en-GB/windows/buy and downloaded the Windows 8 pro program. The good thing is by doing it this way it only costs you £24.99 not the rip off prices from retailers.
I hope this helps and I'm not to late in giving you a solution; I've only just seen your post.