How about another close airport to Destin. You'll probably have to pay the $150/ticket change fee (I doubt you'll get another agent to waive it for you), but you might get a heck of a lot closer at the same or a lower price. How far is New Orleans. They have Southwest service, so the tickets can be pretty cheap. Looks like $325 on your dates

Edited to add: I just mapped it and NO is a bit further than I thought. Gulfport is an hour closer and approx $450 on your dates. You could still break even with the fees on GPT. It is a 3 hour drive or so from GPT to Destin.