Did you have the 8.1 preview installed? When the preview was released, they said that it would not upgrade to 8.1 final, but 8 to 8.1 final should upgrade as expected.
Has anyone used this version yet to upgrade? I've got an MSDN account and a Surface Pro. I've downloaded the .ISO, but when I try to run the setup to upgrade it doesn't accept my key. I pulled the key with MJB keyfinder. Thanks.