Today the Earth's magnetic field is moving a little bit, but is there going to be a point where the field suddenly radically changes direction? If so, what will be the cause of that? If not, then is the field changing all the time gradually?
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) :
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)
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.
Having worked on this problem myself in my master's thesis, see our paper in geophysical journal international(GJI) at http://ceas.iisc.ernet.in/~bsreeni/gji14.pdf the likely timeframe of this event is about 1000 years as per our calculations.
For us humans this is definitely not instantaneous but for the timeframes relevant to earth's history, this is a very short amount of time. So, don't lose your sleep over it, it won't happen in your lifetime.
Magnetic field exist because of the hot liquid in the external core of Earth moves, that movement of charged particles produces the magnetism. Therefore, the magnetic field can't change at once, it only can change slowly with variations of these inner flows.
I "suppose" that earthquakes might produce small changes in the magnetic field, because seismic waves travel through the Earth core and mantle.