What do you mean by today's rate? If you just found out about a new discount, or received a PIN code, then you can try to have them apply it to your reservation. However, rooms need to be available under that discount for it to be applied, if there aren't any rooms, then they won't be able to apply it. Depending on how you made you booking (online or on the phone) and the type of reservation (room only vs package) would affect what the cancellation window is. If you are in the cancellation window you are still able to modify your reservation, you may be required to pay a change fee. If the resulting change in your reservation ends up costing more (IE you are upgrading resorts, adding tickets, etc) then they would typically waive the change fee. You would need to see what the price difference is and if it is worth making the change if the fee is assessed.
If by today's rate you mean that the rack rate for a stay today is less than the rack rate you are paying, then no they won't apply it.