If you search a bit, you'll find a way to do an "in-place upgrade" from the RC to final, copying the CD to a writable drive, altering the content of some files, and installing from the writable drive. That method is not "supported" by Microsoft, and it probably violates the EULA provision that prohibits "working around" the software's technical limitations. But it does require a new CD key, so Microsoft probably doesn't care, if you're willing to assume risks to avoid having to do a clean install.
So far, I have not run across any method for just changing an activation code, and I doubt that there is one. What you have is the RC, and it does have differences from the final. Also, there would be no "maybe" about violating the EULA, and probably federal copyright law. I assume any advice on how to do that would be taken down from here.
If I am running Windows ultimate 7 RC, can't I just change the activation Code for a Genuine Ultimate Version, rather than Having to install everything again and starting from scratch?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic