Well, they don't have a published offer like easyjet (where on your return leg you can take one flight earlier at no extra if you show up at the airport in time and a seat is available). But, hey, a friendly question at the ticket counter won't hurt anyone, if you're at the airport and have time to kill. I've been lucky that way with Austrian in ZRH (weather was not very good that day in January, might have influenced their decision; plus, I didn't have any luggage to check in). But you definitely shouldn't count on taking that earlier flight.