Changing the magnet can do several things depending on the magnet you’re replacing it with. If the magnet is stronger than the replaced one the output can be greater and brighter sounding. If the magnet is weaker than the original, the output can be less and the sound will be warmer. Often remagnetizing a row of single magnets or a bar magnet will make the sound brighter and harder sounding. If the magnetic field is too strong the notes can sometimes sound like glass shattering. Putting in strong ceramic magnets will make the pickup louder and brighter but will also dampen the string vibration and sustain. The strings can stick to the pole pieces when doing dive bombs with your whammy bar. I like the tone you get with weaker magnets and I spend much of my work week calibrating magnets for a desired tone.