Well, I blogged some time ago about how the upgrade from Windows 7 beta to Windows 7 RC will not be seamless. See below:
http://owsug.ca/blogs/brad/archive/2009/04/16/Upgrade-from-Windows-7-Beta-to-RC-will-not-be-Seamless.aspx#comments
Now I actually tested the upgrade path by modifying the cversion.ini file. I posted the process on techRepulic. See below:
http://blogs.techrepublic.com.com/networking/?p=1499
Please back your system up before attempting this and note that the procedure is NOT supported by Microsoft but I got it to work nicely.