By pole shift, you mean a shift in the magnetic poles.
The pole shifts have long been documented from studying the magnetic fields locked into iron-ore formations. From those, we know that an actual pole shift even takes decades at least to happen. During the pole shift, the strength of the overall planetary magnetic field decreases until it's around zero, and soon enough the polarity reverses. Then the field grows in strength up from zero, but reversed over the previous epoch.
It is not sudden and it is not catastrophic. FAIL WILE BAKD.