...he didnt shapeshift...he inhabited a human form, which he has done before as the master from "the Five Doctors" has inhabited that body(human i think) and they offer him a new set of regenerations, plus every doctor is fundementally different so that one could have been half human, one incarnation is alergic to asprin, another to a particular gas...hence the stick of celery (its turns purple if the gas is around and is the antidote to)
Wikipedia is a good place for info
Wikipedia is a good place for info