If you buy an upgrade key, you can't use it with any iso's that have the ei.cfg. I edited the iso the upgrade app created to add the ei.cfg so I didn't have to activate it during the install (to play around) and now when you try to activate it with the upgrade key it doesn't work. I suppose you can try to use an unedited upgrade iso to do a clean install (not sure yet).
I installed using an RTM disk, activated, no problem. Booted from media. I even went back and double checked my install media for size and .wim vs .esd
I think you have to insert key manually (slmgr /ipk key#) and reboot. Did you try this option? I have to test myself later.....
I might have to try that. Does the RTM iso let you activate later? yeah, I tried that. Got the following error: If you run the command you get this: