Ah, you see, with the different incarnations of the Doctor, it's a slightly different story. They're essentially different people in terms of personality and looks - the only thing that doesn't change is their memories. Even the Doctor's opinion about fruit or some other mundane thing changes when he regenerates. I understand where you're coming from, but the mechanics of Doctor Who canon mean that it isn't as much of a problem with Ten and Nine (and any other number Doc) as it would be for say, lawyer!Phoenix and Hobo!Phoenix, or Crisis Core!Sephiroth and FF7!Sephiroth.