According to this post on the Engineering Windows 7 blog you won’t be able to perform an upgrade from the Windows 7 Beta to Windows 7 RC (or RTM) without a little bit of fiddling. This is primarily to encourage people to perform upgrades from Vista or fresh installs, which Microsoft wants people to thoroughly test (because that’s what people will be doing in the real world) rather than upgrades pre-release versions of Windows 7 (which only apply to beta and RC users).

If you are running Windows 7 Beta and you try to upgrade to Windows 7 RC straight from the installation media, you’ll get an error telling you that this is not possible. Similarly, if you are trying to upgrade from Windows 7 Beta or RC to Windows 7 RTM, you’ll get a similar message.

The upgrade is possible. You just have to do a bit of mucking around. To perform the upgrade, do the following:

  1. Copy the contents of the ISO or DVD to a separate storage location (such as a bootable USB storage device, a separate partition or whatever)
  2. Open the file \sources\cversion.ini in a text editor and modify the MinClient number to a number lower than the version Windows 7 that you want to upgrade from – so for the beta you would set this to something like 6900.
  3. Save the file and then run the install routine from the location with the modified file