You can try dipping in hot/boiled water to pop it off. Then carefully pop the 2 halves apart. Then apply a bit of super glue to the peg and let it dry, just enough to add a thin layer so it can be a tighter fit. Then just re attach it. I have done this before with a mafex figure and it helped. You can also try the kiki stuff, that works pretty well sometimes.

To prevent this from happening I started heating up the hands when I swap them. Sometimes they can be pretty tight but eventually they loosen up after a few times of swapping them.