Timeshares don't work the way hotels do.
Even members can't upgrade a room with cash, if the reservation was made with points. At DVC, as with most timeshares, upgrades only happen if the room you originally booked is not usable for some reason (maintenance problems, generally), and there's no other comparable room to put you into. They're very rare occurrences.
Just as an aside, there have been situations where a renter has asked for an upgrade at the desk, and somehow the additional points were taken out of the member's account and used for the upgrade. Needless to say, that should never happen, and is entirely the fault of the CM. But, it still leaves the member without points that they likely intended to use for their own family's vacation, and I'm sure they insisted that the renter pay the additional cost of the points they used.
In short, if you rent points, expect that you will be getting exactly the room type that you booked.