switching WDW tickets for DL ones

[grannyminnie]

I have a question I think has been recently discussed on here somewhere; but not being able to find it, I'll ask again...who do you call or ask about switching unused WDW tickets (on a multi-day pass with the no expiring option) to tickets for DL? The posters discussing it said it could be done, and that they had spoken with a CM about it. Can you just wait and do it when you arrive at DL? Would the concierge desk at the DLH take care of that for you?
Just planning ahead..