Check out http://windows7news.com/2009/04/27/upgrading-from-windows-7-rc-to-final/ for a fairly logical and believable answer to the question of upgrading from RC to RTM.
The pertinent text is shown below:
The only official indicator that upgrading from Windows 7 release candidate to Windows 7...
backup
beta
bootable drive
build number
burning iso
cversion.ini
engineering blog
installation
iso
modification
msdn
release candidate
rtm
technical guide
upgradeupgradesteps
windows 7
windows news
