So would you want to change a doctors role from being a doctor to a construction worker over night and expect him to do the same work as the worker? Hello Bioware has made our companions actually have lives, they aren't just some blank slate that you can change on a whim it gives them more personality and the like. They are fine the way they are, and like it has been said countless times your going to be gimping yourself if you kill your companion.
i'm not saying that it has to be an easy process to change our companions roles. i just wish that the opportunity was there. i know i'm not the only one that hates a companion or two but you're forced to use them because of how the system is set up.
for example you could have a quest line that involves one of your companions to go through some "training" to change their role. bioware is the king of story telling i know they could do it. also make it where you can't change their roles all willy nilly. there could be a 30 day cooldown on it or something or better yet, you can only do it once per companion forever.
as i've stated before if i choose to gimp myself then that's on me and no one else. i'm a big boy and take responsibility for my actions. so if given the opportunity i would gimp myself in a new york minute if i could kill this companion.