Official published rules:

As a Flying Blue Ultimate member, you may change your flight (to an earlier or later departure) free of charge on the day of departure. This benefit only applies on flights marketed and operated by Air France or KLM.

Rebooking is done via the Ultimate Assistant and certain conditions apply:

• You can request a change on the day of departure until 60 minutes before the originally scheduled departure of the new flight. Subject to availability in the original cabin

• Free same day changes only apply at segment level, one direction at a time, and cannot be used to change your itinerary. For example, if original itinerary contains CDG-AMS-JFK, this benefit cannot be used to change to a CDG-JFK direct flight.

• A flight change only applies to flights on the same calendar day and will apply to all travellers in your booking

• If you change your flight, added extras e.g. pre-selected seats or meals, cannot be guaranteed on your new flight

• Change of flight is not possible if you already have checked in a bag

• Original ticketing rules will apply once the scheduled departure time has elapsed

I see no limitations to change on intercontinental flights. To keep the seat, just make like me, i.e. add extra flight to your PNR via PA. On midnight make a call and ask to change the flight. As correctly mentioned above, this should only be a direct flight, e.g. MIA-CDG (not MIA-LHR-CDG).