This will be filled with these themed pieces soon! But for now...
This is basically my favorite version of any kind of swap. Not complete, just a part has been swapped over. An example is
Tops & Bottoms, a special feature from Austin Powers in Goldmember.
Another bonus point for me is if the body part isnt swapped, but instead control is taken, like this one called
Tadashii Majutsu no Asobikata, in which the control of a guys arms is swapped with what seems to be a driving instructor. The
full version has many various parts being swapped, inanimate and person to person. Here is a sample piece that I "translated". Sadly, I could not locate a real one, so enjoy this single page: