RT 8 to 8.1
MS claims everything comes across, however i would still backup, especially important game saves.
This is even more important when your games are setup on a non-system drive (like mine all are). Many files pertaining to those games will be on your system drive that is being updated. It can issues, as it did for me. http://windows.microsoft.com/en-us/windows-8/update-from-windows-8-tutorial