I've just logged into my account, went to the flight I wanted to change and click change. Rebook the same flight and look for the credit to be applied.
Just make sure you are looking at apples to apples. Make sure you're rebooking the same number of tickets as you originally purchased.
Congrats. I spend alot of time on the phone with JB and have to say that they have the best customer service folks. My airfare for JB went up instead of down, but it's OK. I have lots of time for it to change.
Your credit expires 1 year from the day you receive it. One of the nice things is is that you can use it right before it expires and the flight can be longer than a year...make sense?