Is it a bounceback package? If it is, the regular CMs can’t make changes if the offer is expired. Call the bounceback number for changes to a bounceback reservation. They can make changes even if the promo is no longer being offered.
As for dropping the night, Disney uses booking algorithms to prevent single nights they would be unable to book. Call & if the regular CM can’t help, ask to speak to guest services. They have the ability to over ride these systems & make changes. They don’t have to make the change, but they are the ones who can do it.
Also, package prices are set as long as you don’t make changes. Once you do, any price increases that have happened since you booked will kick in. So if tickets or resort rates have gone up since you booked, you will have to pay the new prices.