Going from RC to Win7 Ultimate is easy: copy the Win7 DVD to the hard disk or a USB stick. In the sources folder, open cversion.ini and change MinClient from 7233.0 to 7100.0 or 7077.0. Save the change and run the setup.
If you're not upgrading to Ultimate (the RC is Ultimate), then you need to diddle with the registry of your installed RC version: go to HKLM\software\microsoft\windows NT\current version, and change EditionID and ProductName to the version you want to downgrade to.
E&OE, no refunds, YMMV, Google is Your Friend, etc.