Using the demag doesn't demag the magnet, but does demag the pole pieces and other stuff around the magnet that does become magnetized over time. Never demag a moving magnet cartridge, only moving coils, and even with that, some manufacturers recommend avoiding this process. Benz, and many others do recommend demag.
I have done demags on well over 100 cartridges without any problems. It has always worked. I did once accidentally demag a MM cartridge and it completely ruined it, so be careful.
The tell-tale signs that it needs to be done is a loss of high frequencies, soft, flat overall sound with a lack of dynamics. No sparkle. Demaging will usually bring all of this back.
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