Can you change companions later on, after you already have one?

Yeah, you should just be able to fire the old one.

Really?

Yes you can and also can have 2 at a time 1 human/ghoul and 1 robot/animal at least that's what I remember from reading the game book.

How do you fire your current one?

talk to them, also yeah you can have 2 but one has to be a animal or robot. also those who have the pc version probably will have a mod to let you have as many as you want.