Booking a room only reservation and a package are 2 different things. If Expedia can do it, they would have to cancel the room only, and re-book a package with tickets and dining. It's the same thing you would have to do if you booked directly with Disney. I think the easiest thing would be just to phone Expedia and ask them. Then you'd get your answer.
In the future, I wouldn't recommend booking with Expedia or Travelocity, etc.
I don't think they can get any better discounts then booking directly with WDW, and if WDW releases new discounts for you dates, you may not be able to apply them to your Expedia reservation.