Assuming you booked a Wanna Getaway fare you receive the difference in fare as a travel fund that can be applied towards future travel. When booking a future flight you can apply travel funds if you have the confirmation number of the flight you changed and the name of the passenger is the same on the new flight as the flight you changed. Travel on the new flight must be completed within one year of purchase of the original flight that you changed. If you booked a different fare type you will see a refund to the original form of payment.
You can view travel fund value and expiration dates here:
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