I did the Win 8 Pro upgrade on my mothers laptop. I installed an SSD and reloaded her restore discs, then followed the Win 8 Upgrade Assistant, and chose to keep all files and settings. It only asked for the Win 8 key and not a Win 7 key. After the upgrade, there were a few quirks and oddities, but once I went into disc cleanup and selected to delete all files from the previous install, it has ran great since.
A different scenario I just encountered was a tower i built over the weekend for a friend that had the Win 8 Upgrade license. It was going on a brand new SSD that didn't have any OS previously. I figured I would try the trick like with Win 7 where you just do the install and then reset when you get to the point of entering your key, and then format and reinstall (the second time it will accept the upgrade license). This has always worked for me for Win7 upgrade licenses. Well, on Win 8, it asks for your license right at the beginning of the install and it accepts it even being an upgrade license. However, once you are completely done and at the desktop, it is THEN that it tells you 'sorry, this is an upgrade license only and cannot be used for clean installs' (clean just meaning that there was never an os on the hard drive, not clean as in you have an os, format, then clean install). So I found a nifty change you can do in your registry to let it activate. He had a Win 7 Ultimate retail key just incase, but this was far quicker and easier.