When you select change flight from your southwest app it will pull up all the flights available for that same day (including the one you already have booked) to see if there has been a price change or another flight that is less expensive you could change to. What I love about this os the prices are listed in terms of the change in cost from what you paid for your current ticket. So, is the flight i already booked has gone down in price it might read -$42, meaning 42 dollars would come back to you as a flight credit if you “rebook” your current flight to the same itinerary. You have to go to a few steps to actually confirm a flight change if you decide to do so, so don’t be afraid you could make an adjustment by accident .
this is not the same as Southwest changing your flight because of cancellation or other change in itinerary.