Replacement animation is a technique by which you swap round parts of the character or the entire character even to create the illusion of animation. This is a technique that is not too widely used... well atleast not for an entire animation however there have been animators who have used the replacement technique for entire animations. One of these animators was George Pal, a Hungarian American animator who made a couple of animations under the title of Puppetoons.
This is one of the Puppetoons called Tubby the Tuba made in 1947. This animation uses the replacement technique which basically means replacing parts or even the whole character in each frame resulting in 100's of puppets needed for just one character.
In these pictures of George Pal's character puppets you can see the many number of heads for each expression and phoneme shape. This is a technique used even to this day e.g Nightmare before Christmas used the replacement technique for the heads and Aardman also used this technique on Wallace for the different mouth phonemes which I had the opportunity to see for myself at the exhibition in Coventry.
In this video we can see how many heads are needed for Jack Skellington, with heads needed for every expression, every mouth shape and every one in between. I think that this saves alot of time then having to sculpt the facial expression each frame which holds a lot of space for error so doing it beforehand is a very efficient way of doing it.