Why would you designate a ?/DH and then at some point in time replace the DH? Is the replacement a better hitter? I think the answer becomes "just like" replacing a DH in the lineup normally, but you definitely have a point. Still the rule indicates a player has 2 positions at one time. The ? is a player/position to be named later, while the DH is the same. Normally, if you had DH for 4, then in the 3rd substituted using DH2, the DH still has eligibility to return (as would 4). With the P/DH situation, once removed as a P to become the DH for someone else - that player cannot enter the game defensively. So if you have 10 players on your roster, P/DH is used, "player 10" enters the game using P/DH as the DH for that player - if any of the 9 defensive players cannot finish the game you cannot use the P/DH to fill that role defensively and unless it's the "player 10", you'd have to take an out somewhere in your lineup (harsh, but it's an easy out). Yes, it's ultra confusing and bound to be messed up many times with varying "interpretations".