Personally without even talking about customization and job builds, if SE were to ever implement sub job specializations — like make your MCH a tank with shield turrets and whatnot or turn your DNC into a healer — then it would mean that some of those subjob specs would be released with the new expansions, and that the brand new things you could level up. They would not replace the originals that would still be there, and with potentially new tweaks and changes and actions all across their level progression. Could even ask you to do some quests or things to unlock those too as you progress in the new content.

That's how I'd tentatively approach it anyway. Just a gut feeling.