I've heard rumors that you'll be able to turn the RC into the RTM with a legitimate key, but I'd assume they will be different build numbers and such, prompting the need for a new install. A fresh install would be recommended, in any case.
I'm going to say that there won't be an official way to upgrade from RC to final commercial version (which I believe is out on October 22nd). That and even if you did manage to upgrade, you might have some leftover beta/RC bits lingering about possibly causing issues with the final bits. Generally for a HTPC, or any machine that has massive storage or archives, is to have the OS installed on one partition or drive, and then put all the data/movies/whatever on other drives/partitions/RAID. This way when you have to reinstall, upgrade, or whatever else, the data is not lost. You can follow on the official Windows 7 RC Support Forum located here: http://tinyurl.com/9fhdl5
Once they get closer to the RTM and official commercial release, I'm sure threads will pop up asking about it, so you may find more there.
keither5150 :
I have Win 7 on 2 machines and am wondering if I can simply buy the key once the evaluation period is done.