After doing the level 10 class quest for your starting class, it will tell you that you can change your class freely. This includes Battle classes, Crafting classes, and Gathering classes. You'll be able to swap between them freely, but to obtain them you have to go the guilds that they're in. Some might need you to wait longer due to not starting in the same town (Such as if a glad wanted to be an archer, they'd have to wait until the quest to see the other towns or go there on foot which takes forever.)