The only one that i know you can get rid of is the main spell/combat ability(magery, discord, wrestle, ect). not sure about rest but when you train one up and it has a special ability(dragon breath, rune corruption, ect) see if you can bring up the special ability menu, if you can't then you can't remove/replace.If i get a chance i'll check when i train one up(busy with invasion so may be awhile)