Not usually. Aran's a special case the one time he almost did 'cause he's a Paras, and, well, don't look those up if you don't want nightmares about mushrooms... But the rest, it's been pretty much painless. They start glowing for a minute or so, and then they grow into their new form.

[The two of them are pretty cute together, aren't they.]

Some of mine just choose not to, for their own reasons.