difficulty in ripping a model, changing the shape, and putting back into a game?

[muchgood]

hey guys. i'm wanting to try and rip a 3d model from a ps2 game, then somewhat alter the shape, and then put it back into the game. basically, i want to replace the head of a character with a dog. it would be static and doesn't have to move at all. does anyone know of instances of people doing this successfully or have any leads about where i could look into something like this? my limited understanding is that it's much more challenging than something like changing a texture, bordering on impossible without high level coding skills.

There's a tutorial right here. do note that it does kind of change depending on the game though.