When you reach the story point when you have to travel to Gridania, You can learn conjurer there. There also will be some side quests that will help You gain a few first levels, and hunting log with Fates should help you reach level 15 easily. That way You could continue doing story quest while being a conjurer (if you really want to switch).

Two jobs are currently only available to arcanists (Scholar - healer and Summoner - DPS). You can learn jobs at level 30 after you leveled required second class.