Retro-regeneration

Retro-regeneration was the process of regenerations being forced in reverse, causing the Time Lord in question to revert to previous incarnations, (COMIC: The Fountains of Forever) known only to be attempted by Time Lords of the First Rank (PROSE: State of Change) though it was not known to be permanent in any cases. (COMIC: Timeslip, The Fountains of Forever, Outrun)

Instances
The Fourth Doctor and his TARDIS were once infected by a space amoeba, which caused him to revert into his third, second, and first incarnations. (COMIC: Timeslip)

The Sixth Doctor was similarly forced to revert to all of his previous incarnations on Terra Nova. (PROSE: State of Change) A similar occurrence took place inside the TARDIS while fending off the Funhouse. (COMIC: Funhouse)

While the Seventh Doctor battled the energies of the Charon, he erupted into flame and momentarily regressed into (at least) his sixth, fifth, fourth, third, and second incarnations. (PROSE: Sky Pirates!)

When Borusa was freed from the Tomb of Rassilon, he emerged in the incarnation he was in during the Sontaran invasion of Gallifrey instead of the incarnation that had been imprisoned by Rassilon. (PROSE: The Eight Doctors)

The Tenth Doctor was once forced to retro-regenerate into his ninth incarnation by an age-regression gun. (COMIC: The Fountains of Forever)

When The Then and the Now attempted to ingest the timeline of the Eleventh Doctor, it caused him to regress through his incarnations to the War Doctor, where the Doctor's resistance of that period of his life halted the ingestion. (COMIC: Outrun) A similar occurrence happened earlier, and the Eleventh Doctor appeared between his first incarnation and the Curator. (COMIC: The Then and the Now) The next time it happened, the Eleventh Doctor partially became his second, third, fifth, seventh, eighth, ninth, tenth incarnations, as well as his twelfth, a future incarnation. (COMIC: Running to Stay Still)

The Twelfth Doctor was hit by energy from a time storm and entered a degeneration cycle, going back from his eleventh incarnation to his first. (AUDIO: The Lost Magic)

Other references
Shortly after witnessing the Ninth Doctor regenerate into the Tenth Doctor, Rose Tyler wanted him to change back and asked him if he could do so, only for the new Doctor to claim that he could not. (TV: Children in Need Special)