Rev Bem (Reverend Behemial Far Traveller) was a regular in the first season and a half of Andromeda, and looked more-or-less like a typical Magog. He makes a guest appearance in the 3rd season episode What Happens to a Rev Deferred?, at the end of which he is physically transformed into something that looks more like a cross between a Magog and a human. He only appears once more, in the 4th season episode Fear Burns Down to Ashes.
Why did he have this transformation? I didn't notice any in-universe explanation (the change is commented on, but in the short screen time afterwards didn't appear to have any significant impact), and it's hard to see what an out-of-universe explanation would be, given that the character had only a single additional episode (e.g. it seems unlikely it would be to improve the ratings of the character).
If the change had occurred around the time that Rev Bem left Andromeda, I would have guessed that it was related to something that was planned for the show and never happened (like so many other things), but this change was introduced a year after he had already left the ship and show.