The Class-Changing Items are introduced in Golden Sun: The Lost Age. When one of these items are equipped, it changes your selected character's class to an all-new class series unique to it. These items are artifacts, so you may sell these items off for some coins if you are not fond of their statistics or Psynergy at first, and then be able to buy them back later. There are only three of these items, and each one is acquired only through proper exploration and puzzle-solving in the dungeons they are found in.
Neither these items or any similar ones appear in Golden Sun: Dark Dawn.
The items in The Lost Age, in order of appearance, are:
Mysterious Card: Changes class to Pierrot. Found in Kandorean Temple.
Trainer's Whip: Changes class to Tamer. Fuond in Yampi Desert.
Tomegathericon: Changes class to Dark Mage. Found in Gabomba Catacombs.