Are you saying that Avis will not allow you to return the car on the original date to the original location? Note that many stations permit you to return the car when they're closed and just drop the keys in a secure box, but of course you don't get a receipt and their can be issues if they later claim that the car has been damaged.

If you're changing the reservation now to try to return the car on a later date, I don't think this should be Avis's problem. You need to figure out the most convenient location for you, either to change the airport or to return the car to a place where you can get to the airport easily using a train or whatever (at your own expense).