they will take the 737 as an acceptable rating when the supply of people rated on relevant types dries up. It is all about cost these days, BA will no longer pay for a rating they don't have to.p
Having said that with enough hours on the 737NG you get a significant discount on both the ground school and the simulator sessions required converting onto the B777. I don't know if the same applies converting from the 737NG or classic onto the B747.