Just found a way to upgrade my Windows 8 RC to RTM without losing files and apps:
Extract the Windows 8 ISO or copy all files of the Windows 8 DVD to your HDD.
Browse to the 'Sources' folder and open the file cversion.ini.
Then find the entries as in the attachment below
This might be interesting, the new activation method of Windows 8 revealed, with screenshots. Seems OEMs can pre-activate the software on the PC
Exclusive: Windows 8 contains new product activation method | MyCE.com