Upgrade from Preview to RTM works! When you get following error message you can just do below simple change.
You can't upgrade this prerelease version of Windows. Go to http://go.microsoft.com/fwlink/?LinkID=142337 to see how to install Windows and keep your files and settings.
Just change one file in the installation directory. I'm using a USB Stick to install Windows 8 and changed the file /sources/cversion.ini to
Before it was twice the value 8508.0.
Then the upgrade from a Windows 8 Preview to RTM worked without any problems. Even after some days of usage no problem discovered. But be aware - this is officially not possible, so you may have issues that aren't known so far...