Sorry I don't have the knowledge to help you here but, I am a weaponsmith and when I talk to my weaponsmith trainer he gives me two options:
1. Would you like to learn new recipies.
2. Would you like to unlearn Weaponsmithing.
So try going to you trainer and seeing if they give you that option. You cant go to a normal leatherworking trainer though you have to go to the specific one. Im really not sure though if this will work like i said i havent gotten a leather worker that high so you can give it a shot and good luck.