"From what I remember of 'The Five Doctors', when Pertwee's Doctor met up with Sarah Jane she made some comment about him actually being *him*. She says something along the lines like 'You became all...' and then points to her teeth and her curly hair. At this point Pertwee finishes the sentence with 'All teeth and curls?' so it's possible he doesn't know what his next regeneration will look like, just that it will involve teeth and curls because of the clues from his old companion."