I have changed 241s a couple of times with BA by phone. I was told that if you want to change the destination but still flying to the same country(zone) then they can just change it. If it's a new zone, eg changing destination from USA to Africa then booking needs to be cancelled and rebooked. However, in my case it was done pretty much instantly over the phone so was not a problem. There were fees/ taxes and charges to pay but not sure exactly what they were.