I remember a number of years ago, we were hearing that in due course, all 2 character airline designators were to be phased out and only the ICAO codes were to be used. This was about 6 years ago now and still this seem not to have come to any fruition at all. Did anyone else hear such a thing?
Also, in terms of what is displayed on public information screens, this information comes from the airport and how the airport and how the airport chooses to administer that information behind the scenes. The flight to which all of this information then relates, is shown in such a manner on the screen.
This is why we sometimes see seemingly "incorrect" or spurious flights displayed on the screens. To the general public, they might seem odd, but a positioning flight or a training flight may appear, when it of course should not, because of the way that the information pertaining to that flight has been coded or administered.