Well... I'm saying it's not possible because that's the truth. I've already tried it several different ways
There are too many differences between the 2 cards. First off (and I found this the hard way) the cores are different. I modified the 6800 Go bios by changing one thing only: the core ID, so the card would think it is an Ultra. Well, took me 45 minutes to blind flash back... so that's a no go right there. Why? It's simple, and that is because the vanilla card has an NV41 core while the Ultra has NV42, despite what the bios and drivers say. Secondly, I didn't even attempt an Ultra flash straight up for a very good reason: the 6800 Ultra uses DDR3 memory while the vanilla does not. I didn't want a completely nonfunctional/unrecoverable card.
But don't take my word for it, you guys can go for it and try it :-P