I booked and paid last month for a coach trip MIA-GUA and discovered 12 hours later that I need to change the time of return flight.
I called the Plat-desk and was told I can do for free 1 change during 24 hours after making the reservation.
I do not know if because I am Platinum.

The changed reservation was $40 cheaper, but I was told I can only get a paper voucher and it will take a few weeks. It came in 3 days.