Set and unique items get renamed on act change, but the stats should remain the same. If the helmet, gloves and boots were set ot unique items, perhaps they were just unequipped? Are they missing, in your inventory, or still on the hero but blacked out?
Did your DK only recently start using that sword? He could have been under the influence of a potion or spell that raised agility, which wore off. If not, perhaps a ring/amulet with an agility bonus was unequipped?
