Anyone having a problem with Windows Anytime Upgrade in 7?

Got a windows 7 Home Premium OEM on an HP laptop here, and the owner got a 7 Professional key from his professor. He puts it in the Windows Anytime Upgrade console app and it accepts the key, but it stops midway through and politely says I can’t finish, tough shit, have a nice day. Problem is, now it won’t allow us to put in any other key. Anyone have this problem? I’m thinking it stores the used key in the registry somewhere so if I find and nuke it would it work or fook everything?