As promised, I am laying it all out for you to explain my method of adapting various heads for swapping. About the only thing I am not covering is Jane figures and Power Team female figures.
Basically, I am covering any type of Barbie head on Svetlana, Joanna Dark, and Cy bodies, and any of their heads on each other's bodies.
So without further ado, lets start by looking at what we're dealing with here...
First picture shows representatives of each style of head we are covering, and their neck hole sizes from smallest to largest. (Note: the Svetlana head has Aki Ross' hair on it)