Speaing Britney, a friend of mine with an A320 rating went to BA for an A320 job, was told at interview that he would definitely be on the A320. So he signed up and when his course started it was on the B777. That was this year. So presumably it IS possible for fleet switches to be made by BA. I also know someone who left a company where he was flying A320 to go to fly longhaul at BA. He was promised B777 and was sent to the Airbus fleet.