If your flights were booked at spiritair.com, you just have to go and click on: Make changes to reservations. Just reselect your flights again. Even if your choosing the same flights, make sure you click the little round circle to reselect your flights. It will tell you that you have a voucher for $(amount). It will ask you for your credit card info again. Make sure you click on the booking button. You then will receive an e-mail with your voucher #. If you make the changes online, you are not charged the $25 fee for changes.
You can't get a credit if the fare is a *SyberSpecial*. But for any other fare you can. My fare was a *Special Value*, and I got the credit.