(Of course, behind the scenes, these changes are because the producers are canny enough to realise that a new actor needs to have changes in the show to reduce the audience's comparing them to the previous actor. By the time a new actor is brought in, the show feels like it needs to change also. ; "change, my dear, and not a moment too soon."
And the regeneration confusion--while it's often a great impetus for plot--is really little more than the writers and new actor writing an in-show explanation for the time they need to figure out how the new paradigm works.)