Ok, I don't know if this will help or not, but here's my attempt to try and assist you (sorry about the stupid pink string thing that is on the carpet, we had a Halloween party this weekend and some girls were in pink outfits, so it must be their fault)!
1 - Basic figure being disassembled the easy way, already a pop off joint head.
2 - Using my trusty Walmart dremel, I "trim" the peg down some, making it a tad shorter if needed, but definitely thinner. Not too thin though as I want to keep some strong plastic intact.
3 - Once that is done, and I have selected the new head to be used, I use a pair of pliers to hold the head in place while I use the dremel to carefully make a hole to hollow out part of the head. I have to be careful here, because one slip and I can easily go too deep with the drill and have it come out the other end of his head, or make the hole too wide and tear up part of the neck.
4 - After a few tries to get a snug fit, I find the right amount of drilling to get the head to fit on the trimmed down neck peg.
5 - With this one, he already has a second head hollowed out courtesy of the clone helmet.
Ready for action!