Classes can only be changed at Alltrades Abbey at Newid Isle. This is not available until after a specific plot point, and the player must find Abbot Jack in the Tower of Trades before he will be available to change a character's vocation.
Once the Sage class is unlocked, characters can learn "Jack's Knack," which enables changing jobs on the field.
Class change mechanics
When changing vocations, the character's equipment will be swapped out to whatever they wore the last time (provided it's still in inventory), otherwise they must be outfitted with gear. Levels are separately tallied for each vocation, and the player can review a character's vocations (and their stats for each) by selecting "Attributes" under the main menu.
After completing the main storyline, Abbot Jack will offer Revocation services to any character that reached level 99. Revocation resets the character back to level 1, allowing them to accumulate more skill points. They must still re-obtain all their spells and stat progression (though seed bonuses remain intact). A commemorative accessory is granted the first time any particular vocation is revocated.
A character can only revocate an individual class up to ten times maximum. If the hero revocates a vocation up to this maximum, a special accolade is presented.