If you have the points, book it, then cancel your original. If you Borrow points to book it, the points remain borrowed.
If the website lets you modify your reservation, rather than booking a new reservation, then work your way though it. Cancellation of the first reservations should come at the end when you have completed the new modifications.
If you cannot make thhee change by using one of the two words listed above, then it is best to call
DVC reservations Line and do it through them.
You might will you be able to do it by canceling your current reservation online, but if you do that and the new reservation has disappeared, then you were kind of up a creek.
There used to be a way to do this, by opening a second web browser that is a different web browser from what you are currently using. In other words if you are using Chrome you could open a second window with Firefox. Then you could make the reservation with Firefox, and get to the screen where you committed the points to pay for it. Then you would cancel the points on the first brower with the first reservation, and refresh the point page on the second reservation in the new browser, and it would show your previous points now available to book. Because you have the page open and the process in progress and you complete it within 20 minutes then this used to work. I have no idea if it still works with the New modifications that Disney has made with the booking website.