Microsoft has put up some instructions in preparation for their Windows 7 RC release (build 7100?). Technically you won’t be able to upgrade from a pre 7100 build to the RC. MS really wants you to upgrade from Vista or do a clean install so they can collect real world data on their install procedures. They did however offer some advice if you Really Really Really Need to upgrade from a pre 7100 build version to the RC. They list a way to backup all your current user settings using Windows Easy Transfer and also hacking one of their install ini files to allow a pre 7100 build to RC upgrade.
Definitely bookmark this post if your upgrading for a pre 7100 build as you will need it no doubt.