How to Edit Custom Characters

First thing's first, go to the Manager menu of the Painter Boss DLC, then navigate to the character you want to edit.

Once you find the character you want to edit, use the Edit Character button indicated at the bottom of the screen as indicated below.

Once you click that, you can use the Reassign button as shown below. If you can't click it, disable the character first then you can press it.
Once pressed you'll be able to choose a character to replace with the workshop character, thereby changing their magic, as well as replacing the character in the game.
Enjoy your newfound power to change characters at will.