Thighs should slide off at the swivel, it's a friction peg. It will take heat but for me the thorax comes apart at the thorax with the ball socket going into the stomach top peg. In theory, the whole thing pops apart, but I wouldn't want to mess with the shoulder system. But you can pop the bicep off, it has a subtle securing mushroom peg. Very hard plastic though, so again exercise caution.
I keep saying exercise caution but in actual play practice this thing is solid as a rock. I just wouldn't want to mess with taking it apart too often.
I used an initial hot water bath on both bucks, mostly because getting the second head on was horrendously tight. Didn't really need to do it for the hands, but it probably helped. Had no issues with the hot water. I took the face plates off but it didn't affect the tattoos or anything.
The heads are ultra tight. I have heard from one person in a YouTube comment who replied to me about that that they dremeled theirs and in Asia that was a common trick. I don't want to do that, so hot water for me.
But I also just settled on the mechanical necks for each right now and left it because I like the texture/contrast.
I have not tried to swap the feet because that gave me concern when I gave it an effort. And the gauntlets do have a subtle mold in them that conforms to the forearm if you have any problems with them sliding off.
Also the big gun does have grooves that snap into the gauntlets for a more secure grip. Some reviewers missed this or didn't understand it. It also looks like those tolerances varied but mine was good.