Retro-regeneration

Retro-regeneration or degeneration 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, TV: The Power of the Doctor) known only to be attempted by Time Lords of the First Rank (PROSE: State of Change) though it was normally a short-term experience. (COMIC: Timeslip, The Fountains of Forever, Outrun)

Instances
Following his presence in the court which tried the Second Doctor, (TV: The War Games) Socra regenerated into a new incarnation (PROSE: Doctor Who and the Doomsday Weapon) only to return to his previous body by the time of the First Omega Crisis. (TV: The Three Doctors)

Reunited with the Third Doctor in the Death Zone, Sarah Jane Smith reminded him of when he changed, only for him to explain that it was yet to happen for him. (TV: The Five Doctors) By one account, this was caused when the Third Doctor briefly began to regenerate after battling a mind parasite known as the Nurazh. The Nurazh attempted to take control of his mind, but was overwhelmed by both the Third and Fourth Doctor's minds co-existing. The parasite parished, healing the Doctor and reverting the regeneration. Jo Grant witnessed this, and described to the Doctor his next incarnation. (PROSE: The Touch of the Nurazh)

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, as his arrival in that pocket dimension exposed him to various mutagenic radiation fields; as a result, his body regressed through his past selves seeking a more stable physiology, but he didn't regenerate because he hadn't actually been injured. (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)

Having regenerated into a child incarnation during the Last Great Time War, the Master regenerated back into the after being caught in a time paradox. (COMIC: Fast Asleep)

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)

Having been subjected to a forced regeneration, the Thirteenth Doctor regenerated into. Though he insisted to Yasmin Khan that he could not change back, the Thirteenth Doctor's persona was able to resist the change inside her own mind. From the outside, Yaz and the Doctor's AI hologram used the regeneration energy of a group of CyberMasters to initiate a "forced degeneration", restoring the Thirteenth Doctor and forcing the Master back to his own body. (TV: The Power of the Doctor)

Other references
During the First Omega Crisis, it took time for the Brigadier to realise that the Second Doctor was genuinely from the past rather than the Third Doctor having changed his face back into the Doctor who he had first met, despite both the Second Doctor and Sergeant Benton's attempts to convince him otherwise. (TV: The Three Doctors)

While explaining to Alice Obiefune how time worked different during the Last Great Time War, the briefly appeared to turn back into. (COMIC: Kill God)

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)

On more than one occasion, the Doctor saw that faces from their past awaited them in the future. Looking into his future through a Tomorrow Window, the Eighth Doctor saw many men in pseudo-Edwardian dress as well as the First Doctor. (PROSE: The Tomorrow Windows) Musing that he may eventually retire and become the curator of the National Gallery, the Eleventh Doctor met with the Curator, who he found to have a familiar face. The Curator told the Doctor that "in years to come", he may find himself revisiting "a few, but just the old favourites". Indeed, as well as the Fourth Doctor, (TV: The Day of the Doctor) the Curator had forms resembling the Sixth (AUDIO: Crossed Lines) and the Eleventh Doctors. (AUDIO: The Keys of Baker Street)

Shortly after enduring a temporary forced regeneration which ended with a degeneration, the Thirteenth Doctor was mortally wounded by. This caused her to regenerate into a form resembling the Tenth Doctor, much to his shock. (TV: The Power of the Doctor)

Behind the scenes

 * A very different meaning of "degeneration" was used in the invalid source Movie Millie, where an incarnation of the Doctor in an alternate version of Doctor Who chased Millie in a way which could be described as "degenerate".