I have not frozen mac and cheese before, but I would think you'd want to undercook the pasta and veggies slightly so they won't get mushy. You could also assemble the dish and then freeze it unbaked, and bake it straight from the freezer. We do this with lasagne all the time and although it takes slightly longer to bake it works well.
I have frozen mac and cheese before, and I froze it unbaked. It comes out great! I'm sure you could also freeze it baked though and then put it in the oven for a half hour on a low temperature to thaw it out.