I was not paying attention at the time but I would ask how so?I agree with you as a practical matter.
However, contract law is very strict. One party cannot modify a contract without the consent of the other party.
I don’t know know the details of the actual contract but at first glance, it appears that Disney was the “slimy” one, trying to strong-arm DVC members to modify the terms of the contract.
They allowed you to buy additional time and if you did not your original agreement stayed in place.