This question rests on a premise that Watch Tower may have, at one time in the past, actually had the spirit, which is one reason that many of us may have initially followed the Watch Tower religion.
Your premise is wrong, therefore the inquiry is not valid.
I converted at age 22, which puts me into the second part of your comment quoted above. I joined the JW's becaus I THOUGHT they taught the truth of the Bible. When I found out it wasn't I left. If you want to, you can think that I was following "a" spirit, I prefer to think I was at a low spot in my life and they got me when I was vulnerable. In my case what changed was something in me, not the Watchtower.