Soft plastic figure headswaps are easy. First cut off the heads from two figures. Try and make the cuts straight in one single cut. Insert a short bit of guitar string into the neck of the torso. Leave two or three millimeters of wire sticking out. Use a pair of plies to hold the guitar string, position it at the center of the neck on the torso and just slowly push the wire into the body of the figure.
I use 8 gauge string and one string will do a lot of figures. Cut it with wire cutters, it's very strong and hard wire so don't use your sprue cutter or you may damage it.
Put a bit of gap filling superglue on the wire. Hold the head so the wire is at the center of the neck on the head and push the head slowly onto the body.
Trim the neck and superglue as needed to make a clean join.