Source: I did my PhD work on the Steens Mountain reversal. It is the Earth's reversal with the most transitional directions recorded in much more reliable volcanic rocks. This was possible because of the huge volume of the Columbia River Basalts that were erupting at that time around 16.7 million years ago. Unfortunately it happened so long ago that direct radioactive dating of its length is impossible with current methods.
Actually the more pressing matter regards animals that employ magnetoreception (birds, bacteria even!) where a change like this might be particularly devastating. This of course assumes that there is not already some undiscovered non-genetic mechanism to help with adjusting, say socially or physiologically. Human technology could adapt in at most a matter of decades, while genetic adaptation would takes orders of magnitude longer.
One, is the question of "how long" does a flip take? There is some evidence that it can take as few as 100 years[1], the previous consensus estimate (2004) was about 7000 years[2]. That is important because the faster it happens the more havoc it is likely to cause. If it happens over thousands of years then humans will adapt to a 'non constant' compass direction and find other ways to navigate. For example, GPS 'north' which can always point you to 0,0 on the globe.
The second thing is the timing. The pole is currently "racing" towards Siberia[3], so it may have already started flipping. At 30 miles a year it would take 416 years to fully flip. That is well within the 7000 year window and only 4x the 100 year window.
It is one of the more interesting confounding influences on climate change as changes in the magnetic field strength and orientation can change the amount of energy that makes it into the atmosphere from the Sun. That works both ways, either reducing the energy (more charged particles deflected) making it colder or increasing the energy (more charged particles impact the atmosphere) making it warmer.
[1] https://news.berkeley.edu/2014/10/14/earths-magnetic-field-c...
[2] https://www.nsf.gov/news/news_summ.jsp?cntn_id=100358
[3] https://www.mnn.com/earth-matters/climate-weather/stories/ma...