Or you could just enter the CD Key given to you when you buy the W7 CD in the RC1. Micro$oft will most likely provide an update to update RC1 to 7, but you need a valid CD Key.
There will no doubt be a hack in a similar method to 7 Beta to RC previously but even that caused far more problems post upgrade than the time saved by clean installing and restoring files by backups. For a stable setup it's by far the best way.
But why on Earth would you update from an RC to an official release? Clean install is always the best option.