As someone who decided to change from CNJ to ACN at lvl 20, I have to tell you that it can be difficult to switch. Hopefully you left enough quests so you're not fate-grinding or dungeon-grinding all the way up.

That said, it is much easier to reach lvl 13 (or 15 if you have to travel) than it is 20. You can easily reach lvl 10 using the 50% exp boost from leveling a higher class and just using hunting logs. You may have to run 3 or 4 FATES, along with class quests, but that is hardly a chore. Use your CNJ abilities to minimize downtime so you can pile on the chain exp bonuses. After that, your hunting log and questing will help you reach lvl13/15 again relatively quickly. The biggest hurdle will be figuring out how to get to the class npc you want to play.