How probable is is that earth magnetic field will change 180 degrees at once?
The magnetic north pole (and presumably the south pole) currently moves around 40km/year, although within a year it loops around, may travel much further than that forwards and backwards. Here is a map of the movement from the past century and a half (source)
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However, during a magnetic reversal, the pole may move up to 10 times faster than that, over a period of less than 100 years. Here's a map showing the relative movements (the numbers are thousands of years ago (kya)) (source)

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The reason for reversals aren't very well understood (according to the previous link), and I'm in no way an expert in the field, but just for some fun speculation, I would wonder whether reversals might be related to this phenomenon. Obviously a liquid outer core is going to operate differently from a screwdriver, but the two systems both appear to be exhibiting a bistable rotation while floating in space.