There are a lot of unknowns regarding magnetic pole reversals. They are thought to occur on the average of every 450,000 years, but a pole reversal during the last ice age that lasted less than 500 years is believed to have occurred.

The poles being reversed is not itself harmful. It would likely be a boon for compass makers! I also wonder what effect it might have on migratory animals that sense the magnetic field to find their direction. The biggest concern about a magnetic pole reversal, though, is that the magnetic field likely weakens during the process, reducing its effect of protecting Earth from the solar wind. Even when the magnetic field is at its maximum strength, a solar flare of sufficient intensity directed at Earth would cause catastrophic damage to the power grid, satellites, and pretty much anything else that is powered by electricity unless the flare is detected and precautions taken. It therefore stands to reason that a weakened magnetic field would allow a direct hit by a solar flare of lesser intensity to have such potentially catastrophic effects.

Still, though, a direct strike by a solar flare is a rare event indeed. Personally, I'm not losing any sleep over the prospect of a catastrophe related to a magnetic pole reversal.

(Edited to specify "reversal" instead of "shift" because the location of the N magnetic pole and the S magnetic pole is constantly drifting.)