No. Once your flight has departed, no changes can be made as the seats you booked operated empty in your absence.
Flight changes can be made up to 2.5 hours before your flight departure time, and you will need to pay a flight change fee along with any difference in price between the original and the new fare, if applicable.
Flexi Plus fare customers can change their booked flight on the day of travel, free of charge, to a flight on the same or different route as their original flight (if a seat is available), but must pay any difference in the fare. If they change to a flight departing one day or more before or after the original flight, they will not have to pay a flight-change fee, but must pay any difference in the fare.