In The Brain of Morbius, the Doctor and Morbius (yet another renegade Time Lord that left Gallifrey eons ago) have a "mind battle" and their past incarnations are shown off. When the first three Doctors are shown backwards, there are other incarnations shown and, by Hinchcliffe and Holmes' intent, they were meant to be the incarnations before the First Doctor. But that muted/ignored/retconned away relatively fast, as historically the First Doctor is the First Doctor, as per The Three Doctors. So, now, these incarnations are either a)potential future incarnations of the Doctor, or b)past incarnations of Morbius. The latter of which makes most sense to me, really.