If you need the points from the current reservation for the new one, meaning you would otherwise have to borrow if you could not use them, then Supersnoop, a member on this site who deserves the genius medal for this one, previously provided the one method that actually works where you can first lock in your new reservation online, then cancel the prior one, and then use the points from that cancelled reservation for the already locked in new one. In other words, you do away with the risk of losing both reservations:
1. You need to cancel old reservation amd make new reservation on separate computers or using a computer and a tablet. (Note, I suspect this may work on one computer using two different browsers but I have not tried that way and thus cannot confirm it.)
2. Go to the old reservation and get to the page where you can cancel it, but don't cancel it yet.
3. On the other computer, go to the first reservation page and choose when and what you want as a new reservation and click continue to get to next page that confirms that it is available. If it is available select it and get to next page where you choose contracts to use and you get message that reservation is now locked in for 20 minutes. Choose contracts for the new reservation even though it does not show points from first reservation so that the Borrow Points icon lights up.
4. Now go back to other computer you set up for the cancellation of the first reservation and cancel it. Do nothing else other than the cancellation process for the reservation. Don't switch to any availability pages or other
DVC pages.
5. After cancelling first reservation on the other computer, return to your new reservation and click on Borrow Points. A screen will come up for you to borrow points. Do nothing on that screen except to click Cancel at the bottom of the screen.
6. Result: you will be returned to your new locked in reservation on the page to choose points and all the points from the first resrvation that was cancelled will now be in the contracts and can be used for the new reservation.
As noted I do not personally know whether this can be done on one computer using different windows. Also, its limitation is defined by the Borrow Points icon. It needs to be activated to work and if, when first setting up the reservation, the number of points you have left before the first reservation is cancelled plus the number you could borrow from the next year is fewer than you need for the new reservation, your Borrow Points icon may not light up.