yes, in order to take advantage of a better Ding fare, you must first cancel the original reservations. I've done this many times before. You have to be quick though because sometimes the Ding fare can sell out while you are cancelling the original reservation. What I usually do is....
*select the new Ding fares you want
*open a second window and go to southwest.com
*go to Travel Tools
*select cancel flight
*enter in confirmation # and name of person on credit card, but do not cancel yet
*go back to the Ding window and double check your flights are still available then go all the way to the page where you need to enter payment information
*go back to cancel window and cancel your reservation
*go to Ding window and where it says pay with SW Ticketless Travel Funds, enter in the confirmation # and name of person on credit card from the reservation you just cancelled.
*the price difference between the two fares will remain as TTF under the original reservation # to be used one year from the original date of purchase.
edited to add ~ this only applies to Ding fares. If you find SW has a sale and their regular fares on cheaper than what you already have booked, all you really need to do is go to Travel Tools, select change reservation and it walks you through the change. It's much easier than changing to a Ding fare, HTH!