Surely it's only the flightpath headings that change with the movement of the magnetic north while the physical flightpath route stays in the same place? i.e. the aircraft are still flying over the same ground track?
It's a lot less hassle for everyone involved in actually running the operation to allow flightpaths to drift and keep the documentation the same, rather than worrying about the differences between the intended (true-north-based) and implemented (magnetic-north-based) flightplan on every day except update-day.
