Yes, I realize that a clean install would be far better, but would it be possible to do a simple upgrade from Beta 7000 to the RC?
Yeap it's possible with a minor text edit of one file. Instructions can be found here:
http://www.blogsdna.com/3083/how-to-upgrade-windows-7-beta-build-7000-to-windows-7-rc-build-7100.htm