Retro-regeneration

Retro-regeneration was the name given by the Tenth Doctor of regenerations being forced in reverse, causing the Time Lord in question to revert to previous incarnations. (COMIC: The Fountains of Forever) It was not known to be permanent in any cases. (COMIC: Timeslip, PROSE: State of Change, COMIC: The Fountains of Forever)

The Fourth Doctor was once infected by a space amoeba, which caused him to revert to his third, second, and first incarnations. (COMIC: Timeslip) The Sixth Doctor was similarly forced to revert to all of his previous incarnations. (PROSE: State of Change) The Tenth Doctor was once forced to retro-regenerate into his ninth self by an age-regression gun. (COMIC: The Fountains of Forever)