The way where in Windows 7 you could change your product key is not working anymore.
Instead, you need to use command line to change your product key in Windows 8:
open cmd.exe as Administrator (go to the start metro screen and search for cmd, then right click on cmd and in the bottom select Run as Administrator)
then type following:
slmgr.vbs -ipk PUT-YOUR-KEY-HERE
then, you should have your new key set properly. You can go ahead and try activation then.