I don't know the answer to your question, but if I were planning, I'd try to book the weekends now (even if I had to pay a deposit). I do know that December weekends are very, very popular with the locals and general public, but I don't know how far ahead they tend to book them
We fly in and I cannot stand the stress of not having a place to stay nailed down before I book the airfare. (It's tough to find good prices out of MSP, so you have to grab them a soons as something reasonable appears). Now that wouldn't keep me from looking for discounts later, but I do need that "security" while I look.
FWIW, we seldom pay cash or move on the weekends unless we are visiting attractions other than Disney (OOPS! Didn't want to admit that. We don't do it very often, LOL). We just look at the cost of a week and forget about the differential.
Best wishes -