Microsoft has asked the users of Windows 7 Beta to step back to Vista and then upgrade to Windows RC which is the upcoming build of Windows 7. You must do a fresh installation of Windows 7 RC rather then upgrading the existing beta.
In their own words,
We’ve also learned that many of you (millions) are running Windows 7 Beta full time. You’re anxious for a refresh. You’ve installed all your applications. You’ve configured and customized the system. You would love to get the RC and quickly upgrade to it from Beta. The RC, however, is about getting breadth coverage to validate the product in real-world scenarios. As a result, we want to encourage you to revert to a Vista image and upgrade or to do a clean install, rather than upgrade the existing Beta. We know that means reinstalling, recustomizing, reconfiguring, and so on. That is a real pain. The reality is that upgrading from one pre-release build to another is not a scenario we want to focus on because it is not something real-world customers will experience. During development we introduce changes in the product (under the hood) that aren’t always compatible with what we call “build-to-build” upgrade.
Windows 7 Release Candidate (RC) build is set to be released by next month ie. May for download to pubic, with an unlimited no. of copies. It will be the last release offered publicly for testing before the final release of Windows 7 (RTM).
Microsoft also advised to:
One other related caution is INSTALL ONLY OFFICIALLY RELEASED BUILDS FROM MICROSOFT.
Source: Delivering a quality upgrade experience [ Windows 7 Engineering Blog ]