You will need to cancel your current package and rebook a whole new package with the discount rate, the new booking may or may not be exactly as the first booking, maybe dates or rooms categories can be different under the discounted promotion, in other words you can no longer just call and ask them to apply X discount to your reservation, Disney will cancel and start all over the process, as far as payments made to the original booking, you will definitely will get a refund as per Disney refund policy, I have not experienced if the transfer your balance toward the new reservation right away.
someone out there that can help with that question?