Second Doctor's change of appearance

Following the Doctor's trial, the Second Doctor was sentenced by the Time Lords to undergo a "change of appearance" into the Third Doctor, (TV: The War Games, COMIC: The Night Walkers, PROSE: Doctor Who and the Auton Invasion, et al.) a process which later sources referred to as a regeneration. (PROSE: Timewyrm: Apocalypse, World Game, AUDIO: The Final Beginning, et al.)

Nature
By early account, this event was termed a "change of appearance". (TV: The War Games, COMIC: The Night Walkers, PROSE: Doctor Who and the Auton Invasion, Lourdwater Cottage Hospital case summary, The Three Doctors, Doctor Who and the Giant Robot)

Later sources clarified this event as a regeneration. (PROSE: Timewyrm: Apocalypse, World Game, AUDIO: The Final Beginning, et al.)

Sentencing & choice of appearance
They noted his affinity to Earth and told him he would be exiled there as well as forced to regenerate. The Time Lords gave the Doctor several choices for his new body but he declined all of them. (TV: The War Games)

At the courtroom
In the public account of the trial, (PROSE: World Game) the Second Doctor, after rejecting the choice of faces offered to him, then lost control of his speech and facial muscles, finding himself babbling and involuntarily gurning. As he fought to regain control of his body, resisting the process, the Doctor found himself in a dark void, orbited by swirling images of his own face. His head then abruptly vanished, leaving him clutching with both hands at the empty space where his face had been, but still able to speak. He exclaimed that he felt "giddy", and told the Time Lords, "No, you can't do this to me! No, no, no..." Finally, the headless Doctor was sent twirling into nothingness. (TV: The War Games) This was, by some accounts, the Doctor's regeneration, (PROSE: Lungbarrow) a typical example of a disciplinary regeneration. (PROSE: How to be a Time Lord)

By one account, the Doctor's life passed before his eyes as he regenerated, thinking of all his past companions from his incarnational lifetime. Just before the Doctor's regeneration ended, his "mind was opened, and his entire past and future shone, cruel and clear, before him" and, realising what he had done on the Panjistri homeworld, he sent a telepathic warning to the Seventh Doctor about the Timewyrm. (PROSE: Timewyrm: Apocalypse)

On Earth
By one sequence of accounts, the Second Doctor was exiled to Earth (COMIC: Action in Exile, et al.) but managed to escape the Time Lords before they could carry out the second half of their sentence. He managed to live for some time on Earth, but was eventually attracted to a mystery involving animated scarecrows

The scarecrows dragged him to his waiting TARDIS, where they forced him to regenerate. During the process, the scarecrows programmed the TARDIS for a final flight. (COMIC: The Night Walkers)

Recovery
During the change of appearance, the Time Lords had edited the Doctor's memory to remove his knowledge of how to work his TARDIS, (TV: Spearhead from Space, The Claws of Axos) restrict what he knew about other alien species (PROSE: The Ambassadors of Death) and events beyond the 1970s. (PROSE: The Devil Goblins from Neptune)

The TARDIS travelled to an English field, where it was found, along with the newly regenerated Doctor, by UNIT. (TV: Spearhead from Space)

Legacy
Later Doctors would remember being executed at the trial. (PROSE: The Eight Doctors; AUDIO: Legend of the Cybermen, Stage Fright)

Other realities
In another universe, the regeneration went differently. (AUDIO: Exile)