I have an FM 7 channel goldbox that I sent in back in the days of conversions, they charged me to narrow band it.
As I recall, the charge was for the Rx upgrade. Even though all the old Futaba FM Tx's were narrow band from the get-go, the factory supplied Rx's were not at the time.
Is it illegal under FCC law or under AMA coverage?
In the USA, it's an FCC Part 95 legality. Of course AMA requires compliance too.