The DVC Reservations people can do it without losing the reservation, because they can make sure that the new reservation is available and then they can put it on hold while they are working on it. Once they have it held, they can go back and cancel the old reservation and take the points from your old reservation to put on the new reservation.
It is their ability to temporarily hold one reservation while working on another, that makes the difference. You cannot do that. You cannot put it on hold, thereby locking it in place temporarily while you go drop your old reservation to free up your points. While doing that you could be losing your old reservation and you could be losing the new reservation. If they are doing it They will make sure that It is still there while they are dropping your old reservation to get the points.