woo hoo, you're off to a really Happy New Year!
Have your conf # handy. go to their website and click on Travel Tools from the tabs at the top.
Then click on Change Air Reservation. It will ask for your conf. # and your name.
It will confirm which ressie you want to change then let you rebook a new flight. You can still keep the same exact flight you already had or you can choose a different one.
On the final page it will tell you how much your credit is for.
Personally, once I do have a credit, I go under travel tools, then click on View Ticketless Travel Funds, enter the conf # and it will tell you the amount of your credit and the exp. date. I print it out so I can save it separately from my travel papers for future use.
If you lose that conf. #, SW cannot track down that credit for you.
Once you do it a few times, it gets easy. Good luck and Happy New Year!!