I just had a same situation today for my flight next month. OGG-SLC (A332) was changed to OGG-SEA-SLC (B757), while the original flight has been showing available. I talked with the Diamond Line, and the agent told me the original flight might be cancelled, because the inventory showed so many empty seats. I don't know if his claim is correct (the flight must be empty if all of the customers were rebooked to another flight), but he reinstated my original flight in F.