Hi Dan,
The Disney Dining Plan is a great value for some guests, but it's certainly not for everyone.
In recent years, Disney has become much more restrictive about not allowing changes or substitutions for elements of the Dining Plan. You might be able to find a sympathetic server that would allow you substitute an appetizer for an entree at a meal, but this is not something you can plan for and not something that you should count on happening regularly, even if the price of the appetizer is less than the price of the entree.
My family rarely gets the Dining Plan because we often like to do things such as getting one or two appetizers instead of an entree, and we don't want to be tempted by the included dessert with every lunch. When you're ordering a la carte, anything goes with food selection. On the Dining Plan, there are rules. It all depends on your dining style as to which makes sense for you.
Please let us know if you have additional questions. We're happy to help.