I’m currently refurbishing some of my older Soldier Story figures and the bodies for these (S1 - S2) have always been a pain. The arms have a poor ratcheting mechanism for the shoulder which makes them droop easily and the proprietary neck peg makes a bit of a pain to move them over to a new body. The newer Easy Simple 2.0 bodies have decent arms and those can be swapped out to the Soldier Story S2 bodies. The Easy Simple arms use a similar mechanism to attach the arm to the shoulder support piece which consists of a spring and screw that hold the arm to the support. Simply swap the Easy simple arm and screw/spring/washer hardware (make sure to use the Easy Simple screw as the Soldier Story one is a bit larger) over to the Soldier Story support and reattach to the body. You will need to use CA glue on the threads of the screw first to prevent the screw backing out over time as the spring compresses quite a bit when you tighten it and naturally pushes against washer and screw. A switch to different screw length and using the Soldier Story spring may work better as well as that has less spring pressure, time will tell if the CA glue will keep the screw in place. The Soldier Story wrist pegs are a pretty close fit for the Easy Simple arms, but if they are a bit loose just put a bit of scotch tape around the peg and they’ll sit securely.
On a similar note if you want to swap your Easy Simple body over to cloth boots it can be a pain as they use a slightly larger foot peg that doesn’t easily fit other feet. The old rubberized/soft feet from the ACE bodies can be used and they fit the Easy Simple pegs really well and are just about the same size as Soldier Story or DAM feet.