Dip them in very hot water, I microwave a cup of water for 2-3 minutes then submerge the head for 10-30 seconds; usually it will just pop off with a bit of pressure. Alternatively you can use a hairdryer for a head, but personally the boil and pop is a much easier method.
Not sure what you're planning, but I swapped Luke onto Ki's body and put Ki on a normal height buck... worked out well although on Ki's outfit I had to cut open the bottom of the pants due to the increased height.