The credit is only for future air travel, it's not a refund. As mentioned you have to have your original booking number and I also believe that the credit has to be used within one year of the original booking date, doesn't matter when the flight is. I ran into this issue before and one of my credits had expired. FYI you can do it multiple times, each time you see it drop go and and modify, I've done it three times with one of my flights in November and I've ended up with over 100 dollars in credit.