I agree the classes are more tied to the story than anything. I mean the main story of the game is Cecil, a Dark Knight with many crimes committed in his past becoming a Paladin, a legendary warrior of light who protect and heals rather than destroys. It was very symbolic and showed the player a level of depth to his character through a physical transformation. It would lose it's story impact if the player could just turn around and make him back into a dark knight or into something else.