This totally depends on how you want transforming to work. Basically, what you'll need is an attack with a common event attached that removes the person who uses the attack from the party, and swaps them with the transformed character.
If you want them to turn back after battle, you'll need a parallel process common event or something that checks to see if the transformed character is in the party, and if he is, swaps him for the other one.
Make sure to put a wait on that parallel process, and perhaps set up a switch on it that you turn on when that character is available to stop it from running all the time.