it can be done, but its a time consuming and professional process - and it wont be cheap - you would need to pay for a photographer, the models and the clothes etc
i think you will struggle to create a photoshoot with 2 other people that will exactly match their heads - light source, grain, angle etc, that can be just put together with the old pics
its not just a case of cutting out a head and sticking it on a body
you would be better off taking individual pictures of them and putting them together in a shot than trying to swap heads.
i often have to do photoshopping for film etc, and if the 2 images are not very similar it is not easy.