Do nothing. You have spoken with Marriott, been advised that you will be contacted within 24 hours to discuss a reservation which is not until January. Wait and see what happens.

While the agent gave incorrect advice, you also called and asked a question which was clearly answered in the terms of your booking. Thus, be pleased if Marriott reinstates your original reservation at the original rate, ecstatic if the lower rate is honored, and chalk it up if you are SOL.