Do you mean the axis of rotation or the magnetic pole?

The axis of rotation changing suddenly is rather unlikely, since it violates the law of physics- specifically conservation of angular momentum. Don't worry about it.

The magnetic pole can certainly change, and scientists aren't exactly sure how quickly it happens. I think there was a Scientific American article recently that went over it, but I don't remember the timeframe they were talking about. I think the isogonic lines start going all higgledy piggledy first, so we'll have plenty of warning that it's going to happen.