Are you thinking about adding dining plan or tickets?
In the past adding the dining plan at a later date could disrupt the discount. Room discounts usually have a book by date, and changes made after could remove that offer. We also ran into this problem trying to add an extra day.
I’m pretty sure adding tickets will not affect anything - but it is easy enough just to buy them separate and add to
MDE, and decent savings to be had via this method as well. Boardwalkticketing and attractiontickets are our favorites.