MS specifically announced that the RC was eligible for upgrade. Of course it expires in June, so be sure to make a Backup Image of your finished Windows 7 install stored externally (and in a primary formatted partition on a second HDD if you want) so you'll never need to install again, just reimage the HDD or a replacement in 15 minutes.
The tutorial Gary gave above will show you how to achieve an Upgrade install without having any prior qualifying OS on the drive if that is ever necessary (which it shouldn't be)
But by booting into the Upgrade disk, it will scan the HDD and see the prior qualifying OS so you can proceed to wipe and format the HDD without worry, to get the cleanest possible install.
There is also an in-place Upgrade option to a lesser version (including from RC) you can try which has worked for several around here: UNAWAVE - Downgrade Ultimate to Home Premium or Professional
If it is sluggish due to upgraded RC being beta, you can always then wipe the drive and install from boot with formatting.