You can just keep checking Southwest's website to see if the fare goes down. Southwest will only issue a credit if the lower fare is available on the exact flights in your itinerary. In other words, if a lower fare becomes available on a different flight, even if it's on the same day, Southwest will not issue a credit.
Because the lowest fare class is already sold out on both of your travel dates, it's very unlikely the fare will go down. In fact, the fare is already higher than you paid and there are not 6 tickets even available now at the lowest fare that is available for 1 ticket.