[The following is for idle speculation and mental recreation.]

A recent issue of Scientific American had an article about the
Earth's magnetic field reversals. The article presents some
plausible explanations of the reversals that have occurred
repeatedly throughout geologic times. It also presents some
indications that another reversal is like to happen soon (in
terms of geological time-scale "soon").

It was interesting to contemplate the consequences of a reversal
on our daily lives.

What is ATC going to use for headings?

You may be going geographically northward but the compass is
reading around 180. Which number do you use to describe
what's actually happening?

Airways are prescribed on magnetic courses.
VOR orientations are aligned to magnetic directions.

I imagine that magnetic reversals don't occur overnight, so the
transition would be even more chaotic. We'd get a magnetic
deviation setting along with the local altimeter setting in
clearances?

Since the scenario may be a thousand years or more in the
future ("tomorrow" on the geologic time-scale), none of us
has to worry about it in the next few days.