Originally Posted by
TedToToe
A third possibility is, at the request of the airport authorities. If there is another inbound flight with a similar number arriving at a similar time, for example.
No, this would only affect the callsign, not the flight number. Many flights already have different callsign/flt no. pairings for this reason. i.e. BAW939 = BA939, but BAW27G = BA727.