Asking this on behalf of a colleague here - I don't know the answer myself, so can only assume someone here does 
This chap has Windows 8 Professional - all legit etc.
Needs to do a system rebuild over the weekend so thought he might as well install Windows 8.1 RTM
So he's gone and grabbed the ISO for 64bit Windows 8.1 (don't know where from - that is his business etc.) and he's turned that into a bootable USB installer.
So he'll be installing this on an empty HD. Does Windows 8.1 prompt for a product key during installation and if so, can he simply enter his existing Windows 8 key at this point?
Of does he chose to skip entering a key during install (if you can?) and then enter his key afterwards?
Of is he looking for an 8.1 RTM key which he can use temporarily for the installation and can then change the key over to his own one?
Cheers forum.

This chap has Windows 8 Professional - all legit etc.
Needs to do a system rebuild over the weekend so thought he might as well install Windows 8.1 RTM
So he's gone and grabbed the ISO for 64bit Windows 8.1 (don't know where from - that is his business etc.) and he's turned that into a bootable USB installer.
So he'll be installing this on an empty HD. Does Windows 8.1 prompt for a product key during installation and if so, can he simply enter his existing Windows 8 key at this point?
Of does he chose to skip entering a key during install (if you can?) and then enter his key afterwards?
Of is he looking for an 8.1 RTM key which he can use temporarily for the installation and can then change the key over to his own one?
Cheers forum.