How to Upgrade the Windows 7 RC to RTM
 

The final version of Windows 7 was released yesterday for MS Technet subscribers, but you can’t upgrade directly from a pre-release version—at least, not without a quick and easy workaround, and we’ve got you covered.

Windows 7 checks whether or not the current version you are running is a pre-release copy, and prevents you from upgrading further. The solution is to edit a file inside the Windows 7 DVD—which you’ll have to extract to the hard drive to proceed.

http://www.howtogeek.com/howto/3075/...s-7-rc-to-rtm/

Same workaround for upgrading Beta to RC, copy contents of install DVD to local HDD, edit cversion.ini under "sources" subfolder, change the MinClient value to 7000.0.