Whenever I've asked to change out sides, I've always had one main plate with whatever I didn't want removed from it and whatever I did want added to it. In fact, they often try to make it look nice, as if it was always meant to be that way. Very occasionally, I'll have an extra side delivered on a small plate or in a bowl if the server couldn't get it with the rest of the meal. I've never, ever been served something that I asked not to have and just been told to ignore it. That would be unacceptable to me.