Did the flight times change slightly as a result? If you compare MMB to the email you should have got with the e-ticket, there may be a time change too, it may only be a few minutes but that can be enough to argue for a change to another service or indirect routing (Austin may be a 747 for example). If not, then you are probably stuck, since BA will say it's the same seat (and physically it is). I don't like 777s either, though the 3 class version is fine for me, but sometimes they are difficult to avoid. It is also possible that there will be a further change - to equipment or timings - between now and departure given that it is such a long way off.