We've changed planes in ATL many, many times. 40 minutes is doable (with the usual provisos about planes being on time) as long as you don't spend time looking in shops or getting something to eat.
Despite its size, ATL is actually easy to get around. There are something like 36 gates on each concourse. If your connecting flight is on the same concourse its probably a 10 minute walk between the furthest gates. If your connecting flight is in a different concourse you go to the tunnel at the center of the concourse and take the train (or walk) to the other concourse. Figure 5 minutes to get to the center of the concourse, 10 minutes to get to the departure concourse and 5 minutes to get to your gate.