Southwest is in Concourse C at ATL, so,you won't have to change terminals. If your originating flight lands on time, 40 minutes is plenty of time to make your connection.
That is a gigantic IF. Where is your flight coming from? If its winter, be concerned with delays more than normal.
A quick SW story. I was supposed to fly out of Akron, OH to Dallas, TX. I picked Akron because it was about 20% cheaper than flying out of Cleveland. Its however, about 90 minutes away. My wife (who wasn't going) took the day off and took me to the airport. I got there to check in we found out that thunderstorms around Tampa, FL (my connecting city) was putting me an hour behind, so I was going to miss my connection to Dallas. Problem was, that was the last SW flight out of Akron for the day. They worked with me a ton, and when I told them I would be willing to fly out of Cleveland (kinda on our way home) she booked me for a flight out of cleveland. We drove to the airport, tried to check in. The SW agent said there was no way I would make the connections that the Akron Agent had set me up on. She changed me again. This time we would end up with an extra stop, but we would get there. While in the air to chicago this time, they rebooked me again... cutting down one of my layovers from over 3 hours to 1 hour. The SW agents worked their tails off (at least 4 of them... agents, not tails!) to get me to Dallas. Didn't cost me a penny more. They found flights and connections, and tried and tried again. She at first found a flight for me connecting through Denver... a long, long way out of the way, she said "I'm going to hold those seats for you, and see if I can do better" Which she did.
The moral of the story. If you get to Atlanta and miss your flight, they will try hard to get you on another SW flight into Orlando. The main reason I chose Delta over SW for my next flight to Orlando is that Delta has flights that go to Orlando, from Atlanta every hour. So if we don't get out on time, I have plenty of options later in the day to get the rest of the way there.