if youve ever played final fantasy tactics its pretty much like that. you can freely change between the classes that you have unlocked (as long as youre not in battle)
there are two exp bars, one for your class and one for your character. character level stays constant no matter what class you are. class level gains exp when you kill monsters as that specific class
so for warlock, you would level up a magician to class level 50 then switch your class to shaman and level that to 50 as well. then when you get your character level to 55 you can unlock warlock and play as that class
EDIT: got ninja'd =(
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