Room only discounts require full payment 6 days before arrival (can't remember). Packages require full payment 45 days before arrival.
If a discount offer comes out BEFORE full payment is due, you can call and have the discount applied. If your trip is already paid for, you will receive a refund to your original form of payment (ie credit card, debit card, gift card, disney rewards).
If the offer comes out AFTER the PIF for packages, then you will have to pay a change fee of $50. My understanding is that there is no fee assessed for room only changes prior to arrival.