Upgrade Windows 7 Beta Build 7000 to Windows 7 RC Build 7100
For those of you who don’t have access to the Windows 7 RC via TechNet/MSDN and are still waiting for the May 5th public release of Windows 7 RC (build 7100), Windows 7 RC is out, but you should know that you won’t be able to upgrade your January build 7000 beta in-place (or any build prior to 7077 for that matter), but you will in fact be asked to perform a clean install. 
Long story short, the Windows 7 RC (7100) will only allow upgrades from build 7077 upwards. For any older builds, running the upgrade from the DVD will simply not work.
The solution for this is pretty simple:
1. Download Windows 7 RC build 7100 .iso 
2. Burn ISO on DVD (or use any ISO file mounting tool) and then copy all files & folders to your HDD
3. Now access the “sources” directory within the location where you copied the Windows 7 files & folders
4. Open the file cversion.ini in your favourite text editor (e.g. Notepad)
5. Replace the minclient value from 7077 to any lower build for e.g 7077 to 7000 (just make sure that the number is equal to or lower than the current build you’re running)
6. Save the file cversion.ini
7. Run the installer from the copied directory
And voila!
The above steps will most likely be required for an RC to RTM upgrade installation as well.
Oh, and on a funny note, this is taken from the RC download page itself:
Remember expiration dates: Please plan ahead for the Beta and RC expiration dates. To avoid interruption, you’ll need to rebuild your test machine using a valid version of Windows before the software expires. Windows will notify you that the expiration process is beginning and two weeks later your PC will begin shutting down every two hours. The Beta expires on August 1, 2009, and bi-hourly shutdowns will begin July 1, 2009. The RC will expire June 1, 2010, and the bi-hourly shutdowns will begin on March 1, 2010. In both cases, you’ll need to rebuild your test PC to replace the OS and reinstall all your programs and data.
