Belinda S. answered 09/11/24
USC School of Cinema Grad
Such a transition prior to CGI was a combination of make-up and great editing, using what software was available to "morph" from one character to another. Knowing that such make-up procedures require tedious hours in the make-up chair, including the transitional phases, they may have used a body double for the final images. Think "Claymation." It requires a lot of time and commitment on the part of the actors, DP and make-up FX people to make it happen. Then, there's the cost of having these people for what could take days working on one scene.