It’s happened to us (wife, daughter (aged 4-9), and myself) probably half a dozen times where we were upgraded to business class on a transpacific flight at the last minute and switched once we got on the plane. There has never been an issue as every seat is an aisle and there are a lot of single travelers that easily switch.
It also happened to us where they switched to a different aircraft causing seat changes, again there was no hassle to change with people.
Surprisingly we have never had the issue in coach as we pre-booked the seats.
So you had a choice, upgrade or pre booked seats in coach,
Yeah, outrage