Technically your right, you can upgrade but according to the MS blog, upgrades from 8.1 preview to 8.1 RTM require re installation of all apps - desktop and metro. Since you have to got to the trouble of re installing everything, you'd be much better off to back up your data and wipe first rather then perform upgrade after upgrade etc. Windows 8.1 Preview: An Enterprise Call to Action
Ruffles is correct. These word games drive me nuts. Upgrade, refresh, reinstall, overlay, etc. The "upgrade" ArnoldC is talking about results in a Windows 8.1 system with no applications and additional junk on the hard disk drive. A clean install is much better.