If a magnet becomes weak that’s used in a pickup, the sound will be less bright and less output. When you re-magnetize a magnet that has become weaker it re-aliens the magnetic domains within the particular magnet you are charging providing your magnetizer has enough ampere turns to fully charge the magnet. Also using a stronger magnet with a horseshoe shape can reverse the magnetic poles and also charge the weaker magnet. This has to be carefully done as it can change the magnetic field within the magnet you are charging as you remove the stronger outside force. As you pull the larger field away, it will want to pull the field in a weaker magnet.

Using a magnetizer with a large DC coil works quite well, but you need to wait a few seconds till the magnetizer becomes stable. The most noticeable effect on the newly charged magnet will be more output and a brighter signal. You will also get more string pull if the magnetized poles are too close to the strings. This can reduce the sustain you have on your instrument, and can overload the input of effect pedals and cause more distortion in your system.