Supersnoops's procedure is as follows. It works only if you would have to borrow some of the points to make the new reservation if you did not cancel the exisitng one.
1. You need two different browers, e.g. Chrome and Internet Explorer (or use two different computers, e.g., a desktop and iPad). Sign into the
DVC Member site with both browsers.
2. On one browser go to your existing reservation, click details, and get to the page where you can start cancellation of the exisitng reservation.
3. On other browser, start to set up your new desired reservation on the first page where you can make reservations and once done click to get to the availability page. If what you want is available, click to get to the next page where your reservation is locked in for 20 minutes and you choose points for the reservation. The points shown will be those left without the ones from the existing reservation. Choose the applicable contract(s) and the Borrow icon should come alive. Click on it to bring up the Borrow window and then do nothing.
3. Return to the other browser and cancel the prior reservation. Do absolutely nothing except the steps needed to complete the cancellation.
4. Return to the other browser and click "Cancel" in the Borrow window. The page will refresh itself and show all your points from the cancelled reservation. Now just continue as you would in choosing contracts and doing all the steps necessary to complete the new reservation within the remainder of the 20 minutes allowed
I have used the method twice and it has worked. Others have also reported success. There have been reports that there may be problems with the system when Poly is involved. That seems to be a continuation of the issue that there have been many problems with the system when Poly is involved, including actual availabilty of Poly not being shown correctly in the check availability tool and, in the reservation section, the system's showing availability and allowing you to to go to the page that locks in the reservation for 20 minutes when, in fact, there is no availability for what you want at Poly, and thus the system at some point before you complete the reservation just sends you back to the first reservation page.