That's what I was referring to when I said that it used to be the disc that determined the install type. There used to actually be upgrade/full install discs...and as long as you knew what you were doing you could use an install key to do a full install.
This is no longer the case since the key itself determines what happens.
This is my last time stating this, and it is fact not supposition. So please do not keep posting your doubts over and over again.
The KEY determines the install type, not the media.
The difference is who still has a base OS install that is eligible for upgrade vs those who do not. NOTHING prevents a clean install using those keys as long as there is still a base OS installed. You can reboot and do a completely clean install with the upgrade discs...as long as there is a base OS still present on the HD...you can even wipe the HD once it verifies an eligible license is present.
That's not how it works with Upgrade Assistant. I got Retail disc from TechNet, I installed it and activated with the key I got from upgrade assistant. I installed on a NEW empty SSD, that computer had no other drives.
What seems to determine if it activates it succesfully is one registry key. When doing a clean install, by default that registry key is 0 when installing from Retail disc and activates ok, if installed from upgrade media that key is 1 by default and your activation fails. Changing that key to 0 when installed on upgrade media activates it without errors.
I think that the keys that come trough upgrade assistant are actually full retail keys, not upgrade. Upgrade media that it downloads, limits if it can be used as a clean install or not. Dunno why they did the upgrade assistant like this.
But it's true that usually it has worked as you stated, this is just how upgrade assistant works, it gives retail keys and it is limited as upgrade on media.