There's two ways of doing this.

1) Set a strict Class => Job progression: This would require level capping Classes to lv30. Meaning you cannot level a class past the lv30 cap, and can only continue leveling to 50 by unlocking the job. Once you unlock the job, that class ceases to exist and instead becomes the job (example: If you level Pugilist to 30 and do the quest to unlock Monk, swapping weapons will not make you a Pugilist, but a Monk instead; the job stones would be moved from the Armoury tabs to the Key Items tab, and would no longer need to be equipped).

2) Remove classes entirely: This would require removing all class quests and changing the involved locations. So the Gladiator's Guild would cease to exist, and become something like a training academy for prospective Brass Blades and Sultansworn. The Marauder Guild would crease to exist and become a Warrior training school. And so on. You'd also have to change the current job quests to reflect this. it's considerably more work.