There is not north iron at the top of the molten core and south iron at the bottom of the molten core that will all quickly migrate to the opposite when the magnetic pole shift.
There are iron molecules each with a north and south pole throughout the molten core.
Those molecules north south will shift in place, not all rush from one pole to the other.
The last magnetic pole shift was 700,000 years ago and took 50,000 years from beginning of shift to end of shift.

This was all covered Friday night on TV
2 segments cover magnetic pole shifts and when they've happened and what happens when they do.

The Universe
Magnetic Storm
Season5 Episode 3
It bursts from the sun with the power of ten thousand nuclear weapons... and when it hits our planet, it could create the largest disaster in recorded history. A magnetic storm from the sun could wipe out electrical power, television, radio, military communication, and nearly every piece of electronics in the Northern Hemisphere. It's a "Solar Katrina" -- a planet-wide "hurricane" of magnetic forces that scramble all 21st Century technology, possibly for good. What causes this magnetic superstorm? Why is magnetism so powerful -- and yet so poorly understood? And is there anything we can do to prevent the Magnetic Storm?