I think it matters when you change the park reservation. Same day, probably no issue. Day before might be no issue. A couple days before, may or may not be an issue. Several days before, could well be an issue. A week or more out, most likely will lose the DAS Advanced Selections.
My understanding is that the WDW system does a "sweep" at some point. That point is unknown how often or when, though unlikely to be as frequent as every night or as regular as once a week on Tuesdays -- most likely every 2-3 nights. It sweeps and clears out stray theme park reservations without valid tickets, and it's my understanding that it also can sweep out DAS Advanced Selections without the appropriate theme park reservation. So when you change the reservation in relation to when the sweep(s) occurs likely determines whether or not the Advanced Selection gets lost.
I did it 4 days ahead of time. For what that is worth.