Yes, you do need to uninstall the RC2 first (or any earlier version). However, when you do so, your email folders, bookmarks, and other preferences will be restored when you install the new version.
Bizarre problem on RC2 on my home Win2K box: The email program no longer works. It does not execute, period. I reboot the machine, uninstall and then reinstall RC2, nothing. Sometimes the TaskManager will show mozilla.exe as a running process, but not as a running application. Weird. It's a good thing I don't have anything important on the home email account.. (and the folders are still there, in any case, so I can always just fire up the ol' Linux box and read them off of that)