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
  • Change the value MinClient value to a value lower than one of the RC, so e.g. 8200 and save the file.
  • Now start the installation and pick Upgrade

Further instructions here: Upgrade Windows 8 RC to Windows 8 Pro / Windows 8 Enterprise RTM - Club MyCE