It's an entirely different world with airline airplane avionics. I can speak only in general terms to the Air Bus.
But, I can speak specifically to the Boeing 757/767. Those airplanes came on-line circa 1984-86. No GPS, so their FMS derived position from a combination of inertial and DME trig.
Enter GPS, when sufficiently mature for Boeing, and at Production Number XYZ, the build changed from the "brain dead" FMS to a far more capable FMS that included GPS.
If you had a 757/767 with the earlier FMS, you could "upgrade" for around $1 million per airplane, plus all the lost-revenue down time for retrofit.
Not exactly like changing the Garmin on your dashboard.
