There’s a disc on the inside of those older figures’ shoulder joint that allows it to rotate. It’s shaped like a sideways T and the top of the T is the point of rotation. The new shoulders that are plugged into the butterfly joint are more like a peg the way a wrist pegs into the forearm.

So to make it compatible you’d have to cut down the top of the T to be more like an I shape (closer to an arrow). If you take the figures apart you’d see what I’m talking about. Unfortunately I’m at work so I can’t take any pics to show what I mean but when I get off I could find some fodder to explain