If you need to change airlines while in Atlanta, you will need to go through security again if you have checked baggage. You will also need to transfer terminals if your connecting flight is in a different terminal.
Atlanta's Hartsfield-Jackson International Airport is one of the busiest airports in the world, and it has two terminals: the Domestic Terminal and the International Terminal. Each terminal has several concourses.
To transfer between terminals, you can take the free shuttle service, the Plane Train, which operates 24 hours a day, seven days a week. The Plane Train runs inside the airport and connects all seven concourses of the Domestic Terminal and the International Terminal.
To transfer between airlines, follow the signs for baggage claim and ground transportation, and then look for signs directing you to the relevant terminal or concourse for your connecting flight. If you have any questions or need assistance, you can ask any airport staff member for help.
It's a good idea to allow plenty of time for your connection in Atlanta, as the airport can be quite busy, and it can take some time to navigate between terminals and concourses