Can a player redirect his attack if the defending player concedes?
No, they cannot change who they are attacking. Once a creature has been declared as an attacker you cannot change who (or what) it is attacking.

Say instead of attacking Player B directly you attack their Planeswalker, but before damage gets dealt they Boomerang the Planeswalker back to their hand. You would not be able to redirect your creatures attack to that player, they would just end up ineffectually attacking the air. It is the same thing when a player concedes, your attackers are still going attacking the one player even if they leave the game before the creatures deal damage.

There is also this rule that implies you can still have creatures attacking a player that is no longer in the game.


  800.4e If combat damage would be assigned to a player who has left the game, that damage isn’t assigned.