Changing the callsign and registration

I want to change the callsign and registration of the Singapore 400F (Singcargo and a 9V-registration). Are there any restrictions on how I can do that? Via the aircraft menu in FS9 or direct in the aircraft.cfg? Reason that I ask is that I lost several times my validation on the PAX and I suspect that this had to do with manually installing liveries and changing the aircraft.cfgRob