Originally Posted by
ROCAT
it will mean that it will be done unless it is an issue of safety (weight and balance).
O2 masks (for lap kids) and exit row requirements could also come into play. So could having too many <13 year-old kids for the number of 13+ family members on aircraft with 1x2 or 2x2 seating. That's all I can think of at the moment but there might be more.
Only downside I see is for the passenger who's forced to give up his desired seat in order to make the change. It should be pretty manageable, though.