They're having a fare sale right now, which I think runs through tomorrow (6/8/17).
Here's what I do: After verifying that the price of my flight has gone down, I log on to Southwest, go to My Trips, select the trip that I want to rebook and click Change. That goes to a screen where I can select the specific flight(s) that I want to change (this week, the flight down got cheaper but the return flight did not). Then it goes to a screen that shows all of your flight options and allows you to either select the same flight or change the flight or date, whatever you need to do. The remainder is just like booking a new flight, except that it shows you the amount of money now available as travel funds for future use.
After you have some travel funds set aside, the trick is finding out what you have. There is a
super secret hidden page where you can enter a confirmation number and passenger's name to see the available travel funds.