There won't be a change fee, but there will also be no guarantee that you'll be able to apply the discount code. And whether or not you have a pre-existing reservation will have no bearing on whether or not the code will be applicable for your stay.
Actually, the terms "apply a code" and "apply a discount" do not accurately represent how things work.
As CleveRocks said, there is never a guarantee that you'll receive the discount. There are always a limited number of rooms at each resort that are available for any discount code. If they are gone, they are gone. And sometimes they go really fast. Same way with AAA discounts.
I recall one time that my DU agent tried to get a discount for me, and they were totally gone for my travel dates within a day of their being released.
One of my own personal rules of making Disney room reservations -- Make a reservation that I can live with and am happy with. If a discount opportunity comes along, then that's a bonus.
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