Some of the "buy up to First" offers are actually the cost of buying up to a full fare ficket, which is then eligible for an automatic upgrade, so its not really the same thing as offering first class to a non-elite member for $29 like Northwest used to do. This offer is presented in numerous situations, from the time of ticketing right up until check-in is closed. And I almost paid $120 last week (I was on an H ticket already) for a guaranteed upgrade on a 3-hour flight, then decided to take my chances and received an upgrade after boarding. First was checked in full but there was one mis-connect and I was moved up to First. On the return flight I was upgraded 2 days out.