Just noticed that in the latest SU3 beta, the sim will automatically change the registration base on the cfg file. However it's a little different from 2020, it appears that the "atc_id" should be under "[FLTSIM]"
[Version]
major = 1
minor = 0
[General]
name="Delta N522DZ 'LA28'"
[FLTSIM]
atc_id = N522DZ
atc_airline = Delta
icao_airline = DAL
[Selection]
required_tags = "ext_a359"
[Panel_DynamicParameters]
param.0 = "airline,Delta N522DZ"