Have you ever actually installed an "Upgrade" of Windows? You don't need to install Vista or XP first to use an upgrade version of Windows 7. Indeed, just having an old Windows 7 install on your machine is more than enough.
Which, again, doesn't change the fact that if you already have a Windows 7 install (I don't mean RC, I mean final), you can use the upgrade DVD, blasting the old install away during the process.
