Seats and fares are subject to availability at the time of booking and may be changed or vary at any given moment because of cancellations, modifications or special deals which may be offered for a route.
It is therefore not possible to exchange your existing ticket for a lower cost one and get a refund for the fare difference. Should you wish to proceed with cancellation, you may do so according to the fare rules of your ticket.
Link to Fare rules may be found here