What Oleg just said is about the way I understand it. Before a flip the Earth's magnetic field will become more 'patchy.' There will be multiple poles, and plenty of aurorae.
The Earth is a dynamo whose magnetic material is largely liquid. The changes are all supposed to be a result of change in internal flow. Once that evens out, so does the magnetic field. On a geological timescale, it will happen very quickly. As far as we're concerned, it will Last longer, but still not terribly long.
Stock up on sunscreen.
