They'll simply change the names and nothing else. Think of it like how the job tab in our character menu now lists actually jobs once unlocked instead of the classes.
Anywho, they will never implement branching jobs/classes because it's essentially developing a whole new job. Why have a Machinist who can be a healer when you can just design an actual Chemist to be said healer? That is how FFXIV handles class deviation. You simply switch to another one entirely.
