Upon reaching the age of nine hundred, Dr. Who went through a strange psychological storm, which, not unlike the fabled Phoenix, rejuvinated him into a younger form. Overwhelming him, this force also seemed to give Dr. Who a new personality, although he still retained traces of his "pre-change self." (PROSE: The Phoenix in the Tardis [+]Loading...{"page":"20","1":"The Phoenix in the Tardis (feature)"}) However, other accounts indicated that the process in which Dr. Who wasn't just a rejuvenation but a "change of appearance", the Time Lord's process of changing bodies. (TV: The War Games [+]Loading...["The War Games (TV story)"], etc.)
Dr. Who goes through the "psychological storm". (PROSE: The Phoenix in the Tardis, TV: The Tenth Planet)