Er... I get it that it's about changing the "reference system" and not primarily about GPS. But the difference between "magnetic North" and "true North" depends on your position, and how are you going to get your position if not using GPS?
Using a wide variety of "traditional" techniques that range from dead reckoning (or "pilotage" which is dead reckoning corrected by landmarks) to politely asking a controller if they have you on radar. Airplanes operated under challenging conditions without falling out of the sky for a long time before GPS became a ubiquitous flight instrument (pretty recently, really).
