You could always try the trick that was present in Vista, installing a clean (legit) copy of 7 and say you'll activate it later, then go back and do an "upgrade" install of 7 on top of itself. I remember this worked in Vista, with it seeing the existing, albeit un-activated installation as a valid pre-existing installation and accepting it.
AFAIK there is no real "upgrade" path from Vista to 7, you have to do a clean reinstall regardless. However, I went Vista->7 beta->7 RC->7 OEM, so going Vista->7 Retail might have an actual upgrade option, I don't know.