d. (nsx) If you book a round trip and then before the travel date you find a lower fare in one direction, you have three options:
(1) Use the Change AIr Reservation option in Travel Center at southwest.com or swabiz.com, an option that does not allow you to book a Ding sale fare, (2) Book the lower fare on a new one-way reservation and have a leftover credit on your original round-trip record locator when your travel is complete, or (3) Cancel the entire round trip and rebook both halves, possibly having to pay a higher fare on the unchanged half.
For maximum flexibility to change plans or book a Ding fare, consider booking all your travel as one-way trips. The downside is that if you are not a frequent Southwest flier, booking one-way trips can increase your chance of drawing secondary screening, which is not too bad except that it blocks you from getting your A boarding pass online.
http://www.flyertalk.com/forum/showthread.php?t=422264