Romanadvoratrelundar, better known as Romana, decided to change her appearance voluntarily after the Key of Time adventures. It may seem like a waste of a regeneration cycle, but at least it helps buttress some claims about regeneration made by the Doctor.
It's probably just as well that Romana didn't choose this form, as demi-god DNA must be extremely malleable. Like the Time Lords, they can change their appearance at will (without the constraints of having only a certain number of changes available to them.) The mixture of the two strands of DNA might have caused severe repercussions in the long term.
So she finally settled on a simple humanoid female form and as far as we know, having last seen her in E-Space, she remains in that form even as it ages.
That's easy. We saw it happen within 'Doctor Who'. Princess Astra of Atrios was one of the segments of the Key of Time but in human form*.
1) The Ninth Doctor said that he could have a dog's head or no head at all. So a Time Lord could regenerate into a life form other than a humanoid.
2) The Tenth Doctor claimed that he had fifteen hours in which to lock down the template for the new incarnation. (He demonstrated this by growing a new hand.) By regenerating in the first place Romana was wasting one of her cycles. But trying out the different templates available to her (perhaps thanks to the program installed by Drax?), took place in the span of a few minutes, so no extra regenerations were lost.
3) The Eleventh Doctor said that every regeneration hurt. Maybe that's why Romana did all out of the view of the Doctor (and the audience in the Trueniverse) because it was just too painful and she didn't want to share that. So she slipped into something similar to a changing room.
But in doing so, we never got to see the how the regeneration effect worked for Romana. However, a fan of the show has put together his own version of what it must have looked like:
* I've often wondered if the whole Key of Time story was an inspiration for the Horcrux in the "Harry Potter books. Harry Potter and Astra would then share similarities.