The map of the discrepancies in the Boeing document is very interesting.
The "greater than 10º " changes in the high latitudes are related to how the North Magnetic Pole started racing towards Siberia in the mid-90s.
Revisions are normally only made every 10 years, but the upgrade isn't mandatory - moreover, it normally requires replacing of both ADIRUs for each aircraft, so it represents a significant cost to the operator.
A question for the experts. What is it about how the MagVar database is stored in the ADIRU, that requires a replacement unit? Is the database "knitted" like the ROM in the Apollo Guidance Computer?
