Maybe this is why I'm having such a hard time calibrating my magnetometers :)
I found another viewpoint after a few minutes of searching.
That's easy to fix in software. You will probably have to recalibrate the mag offsets and get updated declinations. Of course that last part requires the USGS and probably NASA, and who knows if the government or civilization will even exist. So you may be SOL on the new declination settings.