I suspect the issue is that DL doesn't want to get into the complexities of reissuing international tickets within T-24 voluntarily, especially given that such tickets are more likely to involve partner segments. At the airport, agents can probably essentially just give you a boarding pass for the new flight you want if there's space, although I would be careful to check that one hasn't been marked as a no show on the original flight, with the remainder of the ticket cancelled.