That’s actually an Excellent question Ed!
Yes you can demagnetize the watch when it’s together. When the balance is out of the movement I like to wrap it with tissue paper because the spring can ‘jump’ and become tangled when being demagnetized. When in the watch the spring is attached at both ends and is much less likely to become tangled.
You’ll find that I probably over emphasize caution in some cases when going through the videos. Better safe when starting out as you don’t need any unnecessary surprises right from the onset. It’s tough enough at times! 
Hope this helps Ed,
Enjoy,
Bob