Not sure about 24 hours before - you can try, but I would think you would stand a better chance when you get to the airport, once the flight has been handed over to airport control.

In the past we have been swapped to bulk heads at check in, if they are confident that they won't be required for bassinets - other times we have been swapped at the gate pre-boarding after everyone has been checked in.

Availability is certainly not guaranteed - if you really want a better seat, the best thing to do is pay the supplement up front.

Nick