Adding another data point, I was on a recent flight where companion and I were 1 and 2 on the list with zero seats but one passenger in First was on a late connection and going to miss the flight. The FA called me to the desk and when she realized my companion was a child said she would need to split our reservations to process the upgrade but she couldn’t do that when she realized my companion was a child.

Not that I would have wanted to be separated from my child in this situation, but just curious—to avoid having to split the PNR, why can’t the agent just override the system and assign one of the pax the seat in F? Does the system prevent them from moving pax from one cabin to another?