By all rights changing to a new class is changing to a new character. The only difference is you're doing it on the same name and model and you don't need to log out.

SE have pretty much discouraged us from creating separate characters. With talk of having advanced jobs (Job + Job = Adv Job), an extra fee to have more than one slot on the same server and the fluidity of the system they have convincingly funneled most players onto one character.

Therein lies the problem though. Once you run through all your quests on one class, you are left with slim pickings the next time around. Some dotted quests around the world, a few story quests and maybe even enough to get you to 15 (which takes no time anyway with Armory buff).

Your options? Grind limited leves (6 per day), grind fates or grind dungeons. Dungeon queues are horrible as DPS and fate grinding is not even really playing the game; it's sitting in town waiting to venture out for 30 seconds to mash some keys.