I asked someone from Tropic about your 'hypothetical' situation and there is no easy answer for the latest booking possible. If there are no bookings at all 24 to 48 hours before an international flight, they can be cancelled. If that were the case, they said it would be very, very rare for the flight to be rescheduled for last minute passengers. On the opposite side of the coin, Tropic has had flights that are fully booked a month or more in advance and, since they use their largest planes on international routes, there is no capacity to seat additional passengers. ...and they don't ever overbook. The aisles are too narrow to haul extra people off the plane.