[t0mas88]

This depends strongly on the region. The US still has a very old system in use for the center controllers which are the big regions. That system wants discrete codes for each aircraft in each area, so they make flights change code if there is overlap while flying into the next region.

In Europe some areas (but not all) have switched to using the Mode-s/ADS-B identification (which is a 24 bits unique code not configurable by the pilot but fixed to the aircraft) in their systems and setting the traditional transponder code to 1000 for all of those aircraft.

In the long term expect everyone to adopt that approach, but things in aviation move very slowly so it will be many many years before we're there.