From what I understand, your advanced class will not be EXP leveled. It will gain it's levels through job specific quests. Your regular classes like PUG, LNC, GLA, CNJ, etc., will still level via EXP.

Personally, I think it's a very interesting concept to level Paladin, Black Mage, White Mage, Dragoon, Warrior, Monk and Bard through means other than repeated EXP grind.

Also from what I gather, you earn the skills for those jobs via the quests as well.