All three prisoners know that B did it. Therefore the odds are not entirely equal because if two of the prisoners team up on the other one they may be able to sway the King to murder the third. Without the tunnel if A and C pointed and swore that B was guilty the King may favour to kill B, all other things being equal.
Now if A switches with B it could sway the odds to kill him as C would still swear B did it but A would be in B's place. So A would marginally increase his chances of being killed if he switched.