Problem:
When I tried to upgrade from Premium to Ultimate using Windows Upgrade Anywhere, I entered a wrong key. After that, the UI does not allow me to re-enter the key anymore. It is back to Premium for each restart.
Solution:
Edit the regedit:
HKey_Current_User\Software\Microsoft\Windows\CurrentVersion\WindowsAnytimeUpgrade
You will find the wrong key there and delete the ProductKey REG_SZ
