I got my dress at David's, as well. And while they say that all sales are final, if you simply need the same dress in a different size, they'll exchange it for you as long as you haven't altered the dress or removed the tags.
If it was a one of a kind or bought from the sales/clearance rack, maybe they can't do anything.
2 sizes sounds reasonable. Much more than that, and the integrity of the dress would probably be compromised. And who knows, if you really did lose that much weight, perhaps the dress would look and fit you totally differently ... and a different style might even look better.
When's your wedding??? 2010?
"... and let us run with endurance the race that is set before us." Heb 12:1
| current weight: 213.0