UPDATE: Same trick can also be used to upgrade from Windows 8 Developer Preview, Consumer Preview or Release Preview versions to Windows 8 RTM version. Just change the MinClient and MinServer values to 7100.
Whenever you try to upgrade from Windows 7 Beta to RC build or RC to final RTM build, you get following error message:
Compatibility Report (saved to desktop)
You can't upgrade this prerelease version of Windows 7. Go online to see how to install Windows 7 and keep your files and settings.
[Close]

Actually Microsoft doesn't recommend upgrading from Windows 7 Beta to RC build and RC to RTM build. So its not possible unless you modify a file "cversion.ini" present in Windows 7 setup folder.
So if you want to upgrade existing Windows 7 Beta to RC or RC to RTM build, follow these simple steps:
1. Go to the folder containing Windows 7 RC or RTM ISO file and extract it using 7-Zip.
2. Now go to "Sources" folder and look for "cversion.ini" file.
3. Open the file in Notepad and you'll see following lines in the file:
Windows 7 RC:

Windows 7 Beta:

4. As you can see the MinClient value is set to 7233 in RC and 7077 in Beta build setup file but the public RC build number was 7100 and public beta build number was 7000. So just change 7233 to 7100 in RC build setup file and 7077 to 7000 in beta build setup file.
Windows 7 RC:

Windows 7 Beta:

5. That's it. Save the file and run the setup.exe file again. Now it'll allow you to upgrade from beta to RC or RC to final RTM build.
Posted by: Vishal Gupta | Categories: Troubleshooting, Windows 7, Windows 8
Windows 8ight
when I tried to do this on win 8 RC it still said I couldn't upgrade. I did the minserver and min clients
Kyle
@Windows 8ight,
I don't use Windows 8 but I think the OS does it differently, despite the minserver and minclients.
Kyle
^^^slightly i meant.
oh cr@p i forgot to read the UPDATE info.