As long as you are a U.S. citizen, there are no exit formalities for the United States, so you just switch planes at LAX. You won't go through immigration and customs until you get to Japan.

You will pass through Japan immigration and customs upon arrival in Japan.

The airline will check your passport at check-in at ATL to ensure it is valid, but there is no emigration control leaving the US.

At LAX you will pass through TSA security just like a domestic flight, or if your departure flight is the same terminal as you arrival flight you'll just walk from one gate to the next.

A few comments:

1. As of this moment, there is no exit immigration control for the US for any citizens of the world. The test for "US Visit" exit procedures have completed and no procedure right now. All foreign visitors with a I-94 need to do is give it back to the airline when checking in.

2. For DL-DL connection at LAX, definitely just walk to new gate.

3. 1.5 hours is plenty of time. If the OP is really worried, they can do a same-day confirmed change ($50/passenger) within 3-hours of the new departure time. It works if you live close enough to ATL. Here is how it works:

You're now scheduled to depart ATL at 1:39p on DL637 (I believe). Not sure which date you're traveling in December, but usually, there are 3 earlier non-stops, some days 4. The previous flight is DL1554, a larger Airbus 330 departing at 11:00a.

What you can do is call DL at 8:00a (3 hours before DL1554 departs) and ask for same day confirmed change. If they have room, they can confirm you on that flight for $50 each. Then you can go to airport right then. If not, then stay home and depart as scheduled.