The magnetic poles do shift. Declination maps (values used to compensate a magnetic bearing to a geographic bearing) are updated (not sure how often but more than every 50 years for sure).
But I don’t see how glaciers melting would interact with magnetic pole movement much which I understood to be all under the crust movement.
To the degree that the actual poles move, it’s going to be a lot less than magnetic pole movement. I’ll have to check out that animation that @Steve_Fernandez linked to above.