If your reason is because of illness or death of yourself or an immediate family member, the Contract of Carriage allows you to make a change without paying the change fee, although you will likely still be responsible for any difference in fare (if any) for your new booking.

If the reason is for other than above, then depending on the severity of the reason, you can always plead your case and hope they'll waive the change fee. Otherwise, the change fee is probably $250 for your flight (though it could be different - a customer service agent can tell you for sure), and you can either: cancel the ticket, with $700 - $250 = $450 available as a credit for a future booking; or, if you already have a date in mind, change the booking to that future date, paying the change fee and any applicable difference in fare.

Hope this helps!