Its true that you cant start the game in one of the "advanced classes"(and i mean this in the sense that you need to advance the story to get to them, not that theyre better or harder to play). With that said, you dont need to make a new character to play a new class. Your conjuror can unlock and level up every class in the game should you choose to. In fact, if you find something you like more then CNJ you can make it your new "main" class by simply leveling that other class up and using it to do story quests once its caught up.