I don't think the top on the Reunion stick unscrews. It seems like I would have noticed that seeing as how I have owned some of those sticks for at least 5 years.
Yes, they do. I am certain of it. I have replaced them before. They are on there pretty good though.
when i built my system, a year and half ago, i was advised that the reunion balltops would unscrew. they seemed very solidly attached. i used rubber to clamp the shaft into a vise and turned the balltop using a large pair of channel-lock pliers lined with rubber. with a lot of effort, i did succeed in turning it, but it did not unscrew; it just turned in place.
but others, like you, are certain that they have unscrewed them. i'd guess that earlier ones were threaded, and later ones are permanently attached.
just to throw in a little curveball here... while i found the reunion to be a great all-around 4-way, its engagement is too slow for defender. betw the the reunion, wico leaf, and ultimarc J-stik (=sanwa JLW), i've settled on the J-stik as the best combination of playability, feel, and long-term maintenance-free operation. as a bonus, it already comes with a larger balltop.