That didn’t work for me? I turned it off, moved a topic, and the ‘watching first post’ user still got a notification. Is there anything else I needed to do?
Edit: I had a brainfart, and didn’t do a bulk move. I’ll try again. 
Update: It still sends the watching first post notification when I use the bulk move too? I’ve tried a few times and I can’t seem to figure out what I’m doing wrong?
I had to do a similar thing a small while back, and I went old school and put a notice up saying that I was moving/going to be moving a chunk of topics from one category to another, and that people who were ‘Watching First Post’ may get a few spam-like notifications. I was hoping it would give people a chance to change their category notifications (or just be aware and ignore them). It wasn’t so bad, though a couple of people missed the notice and were a little confused.
Looking at it again now I know more about the admin settings, it may be possible to default the category notification in the admin settings to normal muted, which would stop the watching first post notifications - but I don’t know how you’d ‘reset’ it to what it was before once you’d finished?
There are a few tips in this older post that might be useful: