You can't mix cash and points on a DVC reservation. So there's no way to pay cash to upgrade a room when you're booked on points.
You couldn't make a reservation change at this point anyway, but for your next trip....if you want club level at Poly, you don't want to rent points. Since there are no club level rooms in the DVC resort at Poly, to get club level in the hotel portion of the resort, the member would need to exchange out of DVC, pay an additional $95 fee, and convert their points to reservation points. Very few members would be willing to do this, David's won't do it at all, and the cost to you would likely be much higher than paying cash and booking through Disney.
The value in DVC (or renting points) is in the use of points at DVC resorts.