Thanks for the input. (1) I understand that the risk was greater that the two diamonds would be discarded than that three spades could, which suggested the greater urgency of the diamond switch. (2) On the other hand, partner figured to have more spades than diamonds (assuming, for the sake of argument, that the 1NT opener has on average 3.25 cards in each suit), so partner might be likelier to hold the spade ace. I was unsure whether the second point was valid, and if it was, how it weighed against the first. The opponents' bidding was bizarre, but that is how they bid it. :) As it happens, partner held ♠AQxxxx ♥xxx ♦x ♣Jxx.