What you can do, is cancel the first reservation and use the credit to buy a new ticket. You will then have some money left which must be used within a year to buy yet another ticket.
Now if you orginal tickets were FULL fare you can just get a refund, but I assume your's were disount.
Hopefully Gail will drop by, she has this changing down to an art and her explanations are much better then mine
For this flight, I had a credit on SW from an earlier trip that I canceled. So when I went to pay I clicked on use Ticketless Travel Funds, put in the confirmation number and checked the funds I wanted to use.