I'm assuming the king already made a choice of A, B or C and told the warden, so that's done and the choice is already made, no need to talk about 1/3 probabilities. The warden won't tell A who the choice was, but he tells him it's not C. That leaves A or B as the person pardoned. It's one or the other, 50/50 chance, so it doesn't matter if A and B switch places unless you add more to the storyline (C rats on B and tells king about the tunnel, king changes his mind, etc.). Furthermore, A and B are identical so there is no way to tell them apart. Doesn't matter if they switch places or not.