You cannot change those codes, but you don't have to.
If you want to create a repaint, and find the code you wanted to use is already taken (which is common when Virtual Airlines are involved), don't use a code, and write the complete name in the Couatl.atc_parking_codes line of the relevant [fltsim] section of all the SIM.CFG that makes your repaint.
Eventually, use the new override parking codes feature GSX, to have your custom airline always appear there, regardless of the scoring.
The only rule, is that you cannot use a comma in the airline name, since that is reserved to separate multiple parking codes for the same airline.
An example of this method can be seen in the default GSX vehicle set: the "Skywest" operator is referred with its full name, rather than a code.
