Yesterday Microsoft released the final version of Windows 7 to Technet and MSDN subscribers, but you can’t upgrade directly from the release candidate to the final release—at least, not without a hack.
Over at the How-To Geek site (my home away from Lifehacker), I’ve written up a guide to the simple process of successfully upgrading from the RC—the trick is a simple hack to the file that controls the version checking. To perform the hack for yourself, simply open up the cversion.ini file in the sources folder and change the MinClient value to 7000. Once you’ve done so, you can upgrade without any issues.
Hit the link for the full walk-through and screenshot tour of the upgrade process. Readers should note that you can only upgrade to the Ultimate edition of Windows 7 RTM—since the release candidate installed Ultimate by default, you can’t use a Home version to upgrade.
How to Upgrade the Windows 7 RC to RTM (Final Release) [How-To Geek]