This is one of those "you can only know if you ask when you get there" type questions. There is nothing within the rules or procedures of the Dining Plan that allow substitution of an(other) appetizer for the entree, but if you ask your server nicely you may be pleasantly surprised with the answer. However, please give the CMs a break. You're asking them for a favor, so please understand if the answer is 'no'.
(One thing to keep in mind about this is that, in general, restaurant appetizers have a much larger labor-cost component than entrees -- the cost to prepare an appetizer is typically a greater percentage of its menu price as compared to the cost to prepare an entree with respect to its menu price.)