the hot glue method for securing a large ball socket head onto a smaller ball neck joint is the simplest method, albeit not necesarilly the prettiest...
Basically you let the hot glue gun heat up some glue and put enough glue into the head's socket to barely fill it... then quickly hold the head on the new neck joint so that the new ball is in the center of the glue. Let the glue cool and then twist the head around to form the new socket. You shuld have a nice tight new socket of glue formed inside the bigger socket. if any glue was pushed out around the socket you can then trim this up to make it look better.
I have had better luck using apoxi sculpt, self hardening clay. Either by the same method of fillin in the hold and pushing in the neck and letting it harden around that OR by filling in the head, letting it harden and then using the dremel to create a new socket much in the way that I did in the video.