Romana I's regeneration

Romana I's regeneration occurred around the time that she and the Fourth Doctor arrived on Skaro in the Movellan Incident. (TV: Destiny of the Daleks, PROSE: The Lying Old Witch in the Wardrobe) She briefly tried on several bodies but was intent on copying the appearance of Prince Astra of Atrios, (TV: Destiny of the Daleks) whom she and the Doctor had recently met while searching for the Key to Time. (TV: The Armageddon Factor)

Motivation
Various accounts gave different reasons for Romana's first regeneration.

One account claimed that she had regenerated purely "for the fun of it". (PROSE: City of Death) The Doctor had noted how oddly casual she seemed with her regeneration. (PROSE: Doctor Who and the Destiny of the Daleks) This cavalier attitude to regeneration was not uncommon among children of Newblood houses such as House Dvora, (PROSE: The Book of the War) Romana's house. (AUDIO: Panacea)

However, other accounts, while agreeing that she voluntarily chose to regenerate, ascribed it a more utilitarian motive: to purge the malicious influence of Pandora, which had begun to resurface after her ordeal with the Shadow. (AUDIO: Lies) Yet another account claimed that the strain of the Key to Time was absorbed into her upon leaving Princess Astra of Atrios, prompting her regeneration; (AUDIO: The Chaos Pool) Romana's later incarnation was unaware of the exact details of her regeneration for most of her life. (AUDIO: The Chaos Pool, Lies)

By another account, the quest for the Key to Time had worn down Romana's body to the point that her exposure to chronodyne, (PROSE: The Lying Old Witch in the Wardrobe) which the Doctor had used to briefly simulate the sixth segment of the Key, (TV: The Armageddon Factor) had started her regeneration. (PROSE: The Lying Old Witch in the Wardrobe)

Physical appearances
During her regeneration, Romana appeared to try out several bodies:
 * one identical to Astra of Atrios, (TV: Destiny of the Daleks, PROSE: Doctor Who and the Destiny of the Daleks, The Lying Old Witch in the Wardrobe) which the Doctor disapproved of because it was a copy of someone they knew; (TV: Destiny of the Daleks, PROSE: Doctor Who and the Destiny of the Daleks)
 * a blue-skinned body (TV: Destiny of the Daleks, PROSE: The Lying Old Witch in the Wardrobe) which Romana "quite like[d]" but complained was "a bit short", (TV: Destiny of the Daleks, PROSE: Doctor Who and the Destiny of the Daleks) wearing a "bizarre, silver, filigreed" outfit (PROSE: The Lying Old Witch in the Wardrobe) including a helmet and cape; (TV: Destiny of the Daleks)
 * by some accounts, an "Amazon" with red hair, wearing a "two-piece outfit covered by a long, see-through robe"; (PROSE: The Lying Old Witch in the Wardrobe, TV: Destiny of the Daleks)
 * a very tall woman (TV: Destiny of the Daleks, PROSE: Doctor Who and the Destiny of the Daleks, The Lying Old Witch in the Wardrobe) wearing "something white and classical" (PROSE: The Lying Old Witch in the Wardrobe, TV: Destiny of the Daleks) which Romana found "very fetching"; (PROSE: The Lying Old Witch in the Wardrobe) and
 * by one account, "an exotic female" of an alien species. (PROSE: Doctor Who and the Destiny of the Daleks)

The Doctor attributed Romana's surprising casualness and level of control over regeneration to her success at the Time Lord Academy. (PROSE: Doctor Who and the Destiny of the Daleks) The Seventh Doctor described this to Bernice Summerfield as "trying out various genetic configurations before she settled on one." (PROSE: Original Sin)

Romana ultimately returned to the form of Astra of Atrios, saying that the body had "looked very good" on Astra. (TV: Destiny of the Daleks) Some accounts referred to this form as Romana's second incarnation; (AUDIO: Warfare, et al.) indeed, by some accounts, a Time Lord's form could remain fluid for multiple hours after their regeneration, going as far as regrowing limbs. (TV: The Christmas Invasion, Let's Kill Hitler) However, other accounts showed that it was possible to regenerate multiple times in a short span of time. (TV: The Curse of Fatal Death, Hell Bent, The Doctor Falls)

The apparent waste of regenerations would have been consistent with the recklessness of Newblood Houses regarding regeneration. (PROSE: The Book of the War) By another account, however, Romana was "still a long way" from the control over regeneration that Gallifreyans in some of the newer Houses demonstrated. In this account, the many forms were assumed by Iraj, a shapeshifter whom Romana met in the TARDIS wardrobe, as a prank on the Doctor. Upon seeing Iraj take the form of Astra, Romana remarked that copying someone else's appearance was "probably against a dozen laws", (PROSE: The Lying Old Witch in the Wardrobe) echoing the Doctor's concerns. (TV: Destiny of the Daleks)

Completion
According to most accounts, Romana had completed her regeneration by the time she finished trying on different forms for the Doctor. They then proceeded to Skaro. (TV: Destiny of the Daleks)

However, according to another account where it was the shapeshifter Iraj who assumed the different forms, Romana I was still in the TARDIS wardrobe, where Iraj then froze her in a stasis. Iraj revealed herself to be the Doctor's TARDIS itself, upset about the Doctor's installation of a randomiser (PROSE: The Lying Old Witch in the Wardrobe) to avoid the Black Guardian (TV: The Armageddon Factor, et al.) and jealous of the Doctor's companions. Iraj, choosing Astra's form because she "could tell" that the Doctor "liked this one", intended to replace Romana permanently.

However, after going on an adventure with the Doctor and Davros (PROSE: The Lying Old Witch in the Wardrobe) where she was taken prisoner by Daleks during the Movellan Incident, (TV: Destiny of the Daleks) Iraj exasperatedly freed Romana, remarking, "I could be the most stunning woman in all the universe, and he'd still only think of me as a particularly clever piece of hand luggage". Romana reassured Iraj that the Doctor cared for the TARDIS most of all, and in return for not telling the Doctor, Iraj helped Romana complete her regeneration. Waking in a body just like Astra's, and hearing the Doctor calling for her (PROSE: The Lying Old Witch in the Wardrobe) to join him in Paris, (TV: City of Death) Romana thought of what Iraj said about the Doctor liking Astra's appearance, and she chose a "schoolgirl outfit". (PROSE: The Lying Old Witch in the Wardrobe)

Behind the scenes

 * While Destiny of the Daleks was mostly written by Terry Nation, Romana's regeneration sequence was written by Douglas Adams, who wrote it as a parody of the Fourth Doctor's post-regeneration costume changes in Robot.
 * Mary Tamm had agreed to play Romana after being convinced she wouldn't just be a "damsel in distress". However, by the end of Season 16, Tamm felt her character had fallen into that category. This, along with the fact her character had no room to grow in such a small format, left Tamm unwilling to carry on, prompting the character's unexpected regeneration in the first story of Season 17. As Tamm was several months pregnant at the time of filming, she couldn't appear in the brief regeneration sequence.