"It is explicit from the terms that Ambassadors are guaranteed one-level upgrade ..."
Actually, even that can be argued...It says upgrade to a "Superior" room, not a "superior" room, the capital "S" being important. "superior" could mean a general term for a better room, however, if "Superior" defines a specific class of room, if someone reserves a "Superior" room to begin with, they would not get any upgrade at all?