I Haven't yet tried to do this, but this is what I would do:
If the new lower rate is not a ding rate, open browser, go to SWA and proceed just like you are booking the flight but under # of people put 8 (the maximum) I do this because it helps me to get a better feel for availability. If lower fares are available for 8 then start over as if booking for your group. Go through until you get to the screen that says apply ticketless travel funds -
Open another browser window, go to SWA - travel center - and cancel your DING reservation. Impatiently wait a couple minutes (and pray) and go back to the browser with the new reservation and apply the ticketless travel funds.
A much safer way, is to book an entirely new reservation on a credit card, then cancel the other. That way you can start thinking about planning another trip to use the funds from the cancelled reservation!
We have DING fares of $55 OW, but I know when there's a special sometimes it's as low as $49. I don't know if I'm going to risk it for $6.