Hi Tenay,
Once you have booked your vacation package, you will not typically be made aware of any special offers or promotions which are offered at a later date and may include the dates of your stay. So once you have booked, my best advice would be to keep checking the
Special Offers section of the Walt Disney World website to check if any offers or promotions have been released.
If they have, you will need to contact Disney on (407) W-DISNEY to ask a Cast Member if it is possible to apply the offer/promotion to your existing reservation. If the Resort and room category you have booked is available, you will be able to apply the discount. If it is not available for your specific Resort selection, you can ask about changing to a different room or Resort category. Most times this can be done without any cost to you (and if there is a small change fee, it is usually offset by the amount of the saving)
I have done this exact same thing myself back in 2009 when free dining was announced without any problems at all.
Wishing you a magical trip filled with memories which will last a lifetime!