Microsoft recently said, those looking to upgrade their Preview versions of Windows 8 to RTM (Release To Manufacturing) bits, won't be allowed.
However, a simple trick made it easy enough to do the upgrade -- "Simply take your ISO/USB/CD/DVD or whatever install media you plan on using and make it writeable in some way.
Find the n:\sources\cversion.ini file on the media and open it with notepad. Change the two numbers from 8508 to 7100 and save and close the file," explains mrdenny.
Now run the installer from within Windows 8 Beta/Cutomer Preview/Developer Preview/Release Preview and you'll be able to uppgrade your Previews bits to Windows 8 RTM.
This trick works well in Windows 7, so should work on Windows 2012 as well.