Angie:
National and Alamo run two rental counters on Disney property: one at the Dolphin hotel and the other at the Car Care Center near the entrance to the Magic Kingdom. I have used the Car Care Center location to return a car in the past simply because its hours were longer, but most importantly, it has its own shuttle that will take you to any park or resort on property.
(They'll also pick you up at your resort if, like you said, you get to Disney and change your mind.)
Just call and schedule your pick-up.
National/Alamo at the Car Care Center
1000 Car Care Drive
Lake Buena Vista , FL 32830
Phone: (407) 824-3470
Hours: Monday through Friday: 6 a.m. to 8 p.m. and Saturday/Sunday: 6 a.m. to 6 p.m.
National/Alamo Desk
Disney Dolphin Resort
1500 Epcot Resort Blvd
Orlando, FL 32830
Phone: (407) 934-4930
Hours: 8:00 a.m. to 4 p.m. every day
Wishing you a flexible, magical vacation!
Whitney