Changing Reversion COMMS
The only reason for a change of callsign is if ATCATC —Air Traffic Control request you do so for reasons of safety; this will normally be because two very similar callsigns are on the frequency and instructions passed to one might be misinterpreted as being for the other. The terminology ATCATC —Air Traffic Control will use is "Change your callsign." You will revert to your old callsign in one of two ways: either the current ATSU tells you to, or you change to another ATSU. The only way you will keep the changed callsign is if your current ATSU has briefed your next controller and he or she specifically tells you to retain the changed callsign after the changeover.
From ICAOICAO —International Civil Aviation Organisation DOCDOC —Designated Operational Coverage 4444: When an ATCATC —Air Traffic Control unit changes the type of call sign of an aircraft that unit shall ensure that the aircraft reverts to the call sign indicated by the flight plan when the aircraft is transferred to another ATCATC —Air Traffic Control unit except when the callsign change has been coordinated between the two units concerned.