Results have varied as reported by a large number of people on the interwebs.....
The general idea is that if you remove the magnetic slide part from the stationary poles, the magnets can loose their strength. Mind you, that is actually very difficult to do.
If you just want to clean the insides and replace the oil, then you shouldn't have any problems.
I've done it with several mag chucks without any change in magnetism....
This is NOT advice, just reported experience.