No sifu here but have recent experience with a magnetized watch.
Bought a Lenovo laptop recently and didn't realise there are 2 very strong magnets located under the palm rest area to keep the lid shut when closed. So, my perfectly accurate Rolex Explorer running at a stable +1 to -1 seconds got magnetized and started running slow and erratically so. It started ticking between -2 to -8 seconds slow per day!
Did the low tech compass check method by moving the watch past a stationary compass and yup, each time the watch moved near the compass the compass hands swung.
So, I headed on to Shopee and bought a China demagnitizing device and received it within a week. Compared some youtube videos to see which was the best method for demagnetizing and decided on the one where you place your watch on the demagnetizer, press and hold the demagnetize button, count to 10 and slowly lift your watch away from the device vertically before releasing the button.
On the first pass, it seemed that my watch was fully demagnetized as the compass hands did not swing that much but nearer the bottom lugs of the watch, the compass still showed some movement. So I removed the watch bracelet and moved the watch sans bracelet across the compass and yes, the watch was indeed free from magnetizm. But the demagnetizing process had actually magnetized the bracelet pin instead which was affecting the compass. So I then demagnetized the bracelet and watch pins separately and did the magnet test again and voila! Watch, bbracelet and pins totally demagnetized.
So depending on the type of demagnetizer you purchase, it might not be powerful enough to demagnetize the entire watch in one single pass and you'll either need to remove the bracelet and do it separately or run multiple demagnetizing passes.
Hope that helps.
Oh, another thing, in the past it's common to find magnetized watches running fast because the maggnetism would cause the magnetized mainspring to bind and give the effect of a shortened mainspring and result in more beats per measure. These days with silicon mainsprings and Rolex parachrom and whatnot, they don't get magnetized but other components still do and this causes more drag on the drivetrain (my layman, non sifu definition) which causes the watch to run slow.