The Windows 7 team has asked that those who have upgraded from Windows Vista to Windows 7 Beta restore your PC to Vista before upgrading to Windows 7 Release Candidate.
On a team blog posting Delivering a quality upgrade experience, the engineering team wrote, “As an extended member of the development team and a participant in the Beta program that has helped us so much, we want to ask that you experience real-world setup and provide us real-world telemetry.”
The team acknowledges the issues involved, “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.”
If there's a business reason to update between Windows 7 Beta to Windows 7 Release Candiate, blog details the steps you can take.
In addition, a direct upgrade to Windows 7 from Windows XP will not be supported. Users can purchase the upgrade version which costs less than a complete new version. There are simply too many changes in how PCs have been configured. Having all of that support carry forth to Windows 7 would not be nearly as high quality as a clean install.
ISVs are preparing for Windows 7 application certification by testing applications on Windows 7 beta. For more information, see John Pelak's posting Win7 Logo Program - an update for ISVs. You can also participate in half day application labs across the country. For more information, see Sign up Now for Windows 7 Compatiblity Labs at a Nearby MTC