I may have found something for this.
look for this folder
E:\MSFS2020\Official\OneStore\[aircraft you want to fly]\SimObjects\Airplanes\[whatever plane]\
(May be different on Steam, I have GamePass version)
Mine for the 152 is: E:\MSFS2020\Official\OneStore\asobo-aircraft-c152\SimObjects\Airplanes\Asobo_C152
Open file aircraft.cfg
Look for line
atc_id = "ASXGS" ; tail number
near the bottom
Change the ASXGS to whatever you want.
Save.
Reload plane (?)(or sim?)
Make a backup of this file or folder just in case. I won’t be responsible for you having to reinstall 91GB
That might keep it from changing back.