The system will not allow it.....

[PAXboy]

That might be the case in the USA, fdcg27. One of the methods of dealing with requests that the airline do not want to deal with is this. You seek a change or an upgrade, a reroute - whatever it might be and you DO ask politely:-

1. At check-in, "You'll have to ask about that at the gate"
2. At the gate, "You'll have to ask about that on board"
3. On Board, "They can only handle that at check-in."
4. Game, Set and Match

Done that - and more than once!!