Grinding the pole pieces on a magnetized and wound coil can cause several problems and I wouldn’t do it. I seen pickup done this way and all the ground and chipped magnet dust clings all over the pickup and poles. The heat from grinding can melt the insulation inside the pickup and later cause the magnet wire to oxidize and break. Grinding the magnet will demagnetize the pole and make it look bad. Just leave it alone and find another pickup that needs rewinding. It can be rebuilt by one of the many pickup builders out there and you’ll have a pickup that works properly.