- Download the Windows 8 RTM ISO file
- Unpack the ISO file to a folder on your computer
- Open the file ..\Win8-unpacked\sources\cversion.ini
- Edit the MinClient value to represent your build (you can get the build with winver.exe)
- Save the file
- Suspend bitlocker protection before running the upgrade (using Control Panel)
- Start the ..\Win8-unpacked\setup.exe and choose upgrade
Happy upgrading to RTM.
Do this "unallowed" upgrade on your own risk. Microsoft have prevented this for a reason but some of us geeks what to do magic stuff and test things that are unallowed.