There is no way to be sure that the aircraft stated on your original reservation will be the one you will be flying on. I used to depend on what I booked was what I'd be flying on. Well, I booked a flight last summer for travel this June - yes, I know...WAY early, but we wanted to use miles - IAD-JFK-ARN. At the time, it was a 767-JFK-ARN. It has since been changed to a 757 - which meant me having to reconfirm seat assignments. Not a big deal, just know that airlines do change aircraft.