I know you can swap cars during a rental, I have done it a few times. But say I'm on a road trip, far away from the original airport I where picked it up. Can I pop in the nearest Avis and swap because, say, the seats are uncomfortable? (and still return to the original location)

Will Avis hit me with any additional fees for something like this? I'm thinking airport locations might be a better choice. Has anyone done something similar?