Another big question. Like limbs, very few heads match from figure to figure. If you’re looking at a ball-and-socket neck joint, the process can be even more complex.
Regarding boil and pop: Many folks will answer this head swapping question with the boil and pop answer (see below). However, this rarely works as most neck joints simply don’t match. Boiling is good for removing a head, but very difficult to add a head. I’ve done it, but even on the simple ones I usually find it to be too much trouble.
To make a simple swivel neck joint: I usually start by simply cutting off the unwanted head of the target base figure, and the wanted head of another figure. If a hole is present in the neck of the target figure, so much the better. If not, I’ll drill a hole into the neck, down into the body. I then drill a shallow hole at the base of the new head. After drilling, I find a piece of styrene rod that very nearly matches the size of the holes, cutting off an inch or so of the rod to be used as a neck peg. The peg is then Superglued into the shallow hole of the head. The head (with new peg) can now be placed into the hole on the neck of the body. If the hole is a bit too large and fit is not snug, you can use masking or Scotch tape to increase the thickness of the styrene peg by wrapping layers around the peg. When you’re done, you have a smooth, functional neck swivel joint!