There is no need to open up the phantom or disassembly of any motors.
Remove the 4 motor screws and carefully lift the motor so its easier to work with. Find an air compressor and blow from top down.
This will move the magnetic sand (ferrite) to the bottom of the motor. Use tweezers or something pointy and nudge the ferrite to the bottom of the neo magnets.
Remove the 4 motor screws and carefully lift the motor so its easier to work with. Find an air compressor and blow from top down.