Per my post above, you can change your secondary class but with the price of Aeria Points. Hopefully in the future, Aeria will give us a free second-class change item. But for now, I believe this is the only way you can change your class.
You need to finish all your main quests at Cactakara Forest. Afterwards, once you finish all of that, you will be sent to talk to Belenus, the Grand Bishop. He will then tell you to talk to Andre, who will allow you to choose a subclass.
If you reset it, does it lose any progress or anything, or do you just choose a new one like that was the one you picked from the beginning? (Other than having to buy a new weapon). I'm just scared of pressing "reset" and it being like "Ok, reset your entire profile thanks byebye."