BRIGHTON, UK — Swapping out pieces in a game of chess is only a smart move provided you hold the most on the board, or at least the strongest position. But a new show at the Barbican in London suggests chess could be a “metaphor of exchange” between the artists it lines up. According to the theory, Duchamp swaps ideas with acolytes: John Cage, Jasper Johns, Merce Cunningham, and Robert Rauschenberg. And yet the Frenchman, superb chess player that he was, came out conceptually on top by the time of his death in 1968.