I did... In order to do a true upgrade install, you would need to mod an .inf file associated with the Upgrade installer. But it's much better to just do a Custom Install (which is what I did). That will dump all of the data from your RC install into the folder C:\Windows.old.
No other discs needed. I installed the RC off a USB drive and activated it. Then, with that USB drive unplugged, I was able to install the Upgrade version using the Custom Install option with just the Upgrade DVD and the Upgrade license code.