Like the mechanical displays at various airports across the world pictured in the article?
Overall seems like a project of huge size and somewhat unknown reach. Not my domain, but I would imagine all kinds of ground support systems to have the four letter flight numbers baked deep into their assumptions. And not only direct support like fuel, cleaning, work shift scheduling, but also logistics systems for cargo, military systems, myriad of ticketing systems etc.
My understanding is that air traffic control is extremely computerised even though direct control is still done over analogue VHF voice radio. The flight numbers are essentially only human-facing (both passenger flight numbers, and ATC callsigns). When flights are handed off between different stations or different flight information regions, their computer representations must conform to the Flight Information Exchange Model (that's the FIXM in the website I linked above).
It's a very top-down initiative by ICAO and all the large aviation authorities—quite unlike the rest of the software development industry.
Frankly speaking ATC is the only thing that really matters in the context of flight safety—ticketing, terminal operations, etc don't (and don't need to) deal with FIXM.
> There is. It's called the Globally Unique Flight Identifier (GUFI), and it's essentially a UUIDv4[1].
Great, step one cleared.
Now have all the legacy systems everywhere that use 4-digit flight numbers been updated or replaced?