If the rules for the ticket you bought say $500 to change the dates then that is going to be the price wherever you make the change. Don't just turn up at the airport - there's a high chance that there will be no availability on the flight. Change the reservation ASAP and pay the $500.

If you want free changes of dates you should have paid more for the ticket in the first place. Cheap tickets come with restriction that's one of the reasons why they are cheap.