It is believed that upgrade from Windows 7 RC version to final release was impossible. However, there are always some enthusiastic geeks working around to make impossible things possible. Thanks to the How-to-Geeks, for they have found a way to install Win 7 upgrade without delete all your RC version data.
Here is the tip.
How to Upgrade the Windows 7 RC to RTM (Final Release) – The How-To Geek