Just found this on Windows 7 Center Forums that Windows 7 Build 7100 Release Candidate for both x86 and x64 versions has been leaked on the Internet. The build string for Windows 7 Build 7100 RC is 7100.0.winmain_win7rc.090421-1700 which means that it was compiled on 21st April 2009.
Microsoft recommends that you do a clean install of Windows 7 RC Build that’s why they have deliberately locked the option to upgrade to Release Candidate Build from previous Builds of Windows 7. If you really want to upgrade to RC from previous Builds of Windows 7, then you can do it on your own risk by following the procedure which the Windows team has posted on their blog here.
Microsoft is expected to release Windows 7 RC Build to public on 5th May 2009.