I couldn’t post this anywhere else for some reason, but for those of you wondering how you can change your tail number and LEAVE it changed, I found a solution.
Make a backup of this file or folder just in case. I won’t be responsible for you having to reinstall 91GB 
Look for this folder in your installation drive/folder:
\Official\OneStore\[aircraft you want to fly]\SimObjects\Airplanes\[whatever plane]\
or, possibly for Steam (unconfirmed)
\Official\Steam\[aircraft you want]\SimObjects\Airplanes\[whatever plane]\
May be different on Steam, I have GamePass version. Anyone with the Steam version, let me know the folder structure and I’ll update this post with that information.
Mine for the 152 is:
E:\MSFS2020\Official\OneStore\asobo-aircraft-c152\SimObjects\Airplanes\Asobo_C152
but others have reported a different location for Game Pass, such as
C:\Users\User Name\AppData\Local\Packages\Microsoft.FlightSimulator_8wekyb3d8bbwe\LocalCache\Packages\Official\OneStore
Open file aircraft.cfg
Look for the following line near the bottom:
atc_id = "ASXGS" ; tail number
Change the ASXGS to whatever you want. I set mine to N721B.
Can also do this with the lines
atc_airline = “”
atc_flight_number = “”
Save.
Reload plane (?)(or sim?)
Upon reloading the plane again, the ATC Option will show an empty tail number as usual, but your tail number will be what you entered into that line in the cfg file.
From u/puckstar on Reddit, you can do this with multiple cfg files at once using Notepad++ and do a search for the files: aircraft.cfg
You should get a lot of results. Now select all of them and right-click and open in Notepad++.
Then do a Replace (Ctrl-H) and replace (for all opened documents!) for instance:
atc_id = “ASXGS” with atc_id = “PCKSTR”.
