As long as you're health enough to walk briskly, ATL is a dead easy airport for connections. The only time an hour D-D or D-I connection can get a little iffy are mornings when half of the small metro East Coast is trying to connect somewhere and there can be delays waiting for a gate and during late May to early September thunderstorm season. (it's the South's version of snow season-something that must occasionally be worked around, but not an everyday thing)
And I've done plenty of 39 minute domestic to domestic morning connections during the summer that have been totally uneventful.