I’m extremely concerned about seat relocation.

We have a major family trip to Japan planned for November. Direct flight from ATL-HND.
Everything for the trip is prepaid and planned out.
We are a family of 5 (3 adults 2 children).

I upgraded us all to Comfort+ with 2 being in row 32AB and the rest of us in a 3 block center bulkhead row 30DEF.

My concern is that because we occupy a bulkhead row we could be relocated if an obese person with a large dog they claim is a service animal tells the gate agent they need more space. Then we could be forcibly downgraded or rebooked on another flight.
This would be a disaster for the trip.
Especially traveling with children.

Is this something common enough to be concerned about? Even missing one flight on this itinerary would be hurtful financially.