with Moiraine's recounting of events, we have this mystery figure(presumably Moridin) entering 'Finnland and negotiating for Lanfear. She had begun to be drained by the 'Finns, so her abilities were weakened. But otherwise she was unchanged.

What happened to give her her new appearence is very likely punishment enacted from the Dark One. Lanfear prided herself on her perfect beauty and grace, traits Cyndane does not share. And Cyndane is old tongue for "Last chance." Considering that she was given this new name intentionally, it is a small leap to consider that her new body was also an intentional move.

So if she was killed and soul-transferred, it is this humble reader's opinion that it was not out of need, but rather out of a calculated move to punish her wayward behavior.