You can use two different approach:
Morph targets: both characters must have the same vertex id and polycount and you can interactively swap materials to have a nice and clean effect
Character swapping: good old “put a particle effect to confuse the viewer” and just swap the skeletal meshes on the fly 
If you’re going to use the “Mechanical look” for the rage version, well, good luck, is going to be a nightmare to create morph target for it 