Yes you can change the ticket, you have up to one year on most airlines. However, you have to pay the fee that the airline charges, which ranges from $35 to $50 plus whatever the difference in cost for the flight you are booking. Thus, a flight you purchased for $300, which is now selling for $350 would cost you $100 more. If the flight costs less, which is unusual, you get no money back. And my experience has been, that they still charge the change fee.