Final Fantasy Hacktics

Hello guys, how do I change a class in the game? I would like to exchange Lancer for a Holy Knight. But when I change the skills, the class is already mastered.
I am adding Sprites to some BMP's I found here in the Forum and found it cool to leave the characters as Classes, as it is not possible to add many main characters due to lack of space in FFTEVGRP. How do I upload the changed sprites here in the Forum? I try but they won't T-T
Thanks and good night (Good morning to you on the other side lol)