Magnetic shifts are mild. Mostly just electronics effected. The polar shift is a lot more dramatic. It would basically move the crust in relationship to the sun. When all the planets line up in 2012 there could be a slight difference in gravitational pull. Which isn't predicted to do much with the present day Earth crust configuration. But if there was an inbalance in the crust, then the theories abound that there will be a shift. If the vulcano erupts, melts just one ice cap, the plants all line up right at that time(2012) there may be enough pull to cause the polar shift. Scientists can prove polar crust shifts in the past but they didn't know why they happened.