Instructions
Open the main menu on the world screen by pressing the "Start" button. Choose "Formation" from the window that appears.
Highlight the character whose job you would like to change by using the direction pad. Press the "Triangle" button to bring up the character menu.
Choose "Change Job" from the menu that appears. This will take you to a list of available jobs.
Find the new job class you would like for your character and select it with the "Circle" button. You can only choose between two job classes at the beginning of the game, the squire and the chemist. More classes become available as you level up the original classes.