Three extra hours, one when you arrive, two when you depart.
Most but not all of the time you can show up early and get your car early with no extra charge. The downside in your case is that you have to bring the car back earlier. You can ask how much more the extra hours will cost when you arrive at the rental counter. If worse comes to worst you may just have to hang around the airport (want to try a restaurant at the Hyatt hotel?) for a few hours.
Caution, not all rental companies let you simply bring the car back late and just pay the extra hour or extra day rate. You may be back charged the entire rental at a higher rate even if you request the extra hours upon arrival at the rental desk. This, together with how sticky they are about showing up late for pickup, depends on how busy the rental agency is.
The extra hour and day rates are guaranteed only when you reserve the car with all of that time included.
So you may still be stuck with re-shopping for a new car rental given that the rates will probably be different when you try to modify your existing reservation.
Disney hints:
http://members.aol.com/ajaynejr/disney.htm