Is it possible/normal to have actors wear a green color/screen mask, and digital replace their heads later?

I've not heard of this happening with heads, but it is certainly possible. Similar things have been done on films, usually with other body parts such as missing limbs (like Lt. Dan in Forrest Gump). In the third Terminator film, which is awful by the way, Schwarzenegger had green patches on his torso which in the finished film were digitally replaced by machinery.





The head would have to be computer generated though. If it was simply imposed from another film it would not match up with the body's movement.