Just to reinforce some of the other responses... no, absolutely 100% you may NOT substitute any first course in exchange for the dessert which the DDP includes.
You may, as indicated above, order an appetizer AS your entree.
You may also order and pay for an appetizer.
You do not need to order or eat dessert just because it's included on the plan you purchase.
You can, for an additional $32 per night per adult and $10 per night per child, instead all purchase the Deluxe Dining Plan which DOES include appetizers.
You can also simply not purchase ANY dining plan, instead paying out of pocket for only and exactly the courses you wish to eat.