Well to put it simply, when you learn how to move and scale things in your 3d modeling program (i.e 3dsmax or blender), you just import the entire HRC of the original field model into it.once that's done, you import your new model, and scale it down and put it aside for easy access later. when that's done, pick a body part. if you're gonna start with the feet, copy one of the feet from the new model, bring it over, and resize it until it's the right size. the feet never normally need tweaking. if it's something more tricky, like a forearm, you do the same as you did with the feet, but you'll notice that on the original, the lower part is a lotter fatter than on the new one, and the higher part a lot slimmer. all you do for this is to just scale and move, sometimes rotating selections of verticies (this requires you to know how to mod things sub-model in your 3d modelling program) until the whole body part has roughly the same shape as the original. that's exactly what i did with my sephiroth chibi. i just spent hours moving and scaling verticies. (i really need a life, lol)
you can never have it too much like the original, because the new one is smoothed and textured, something the originals never had. also, the new models will probably have a lot more verticies, making things a lot smoother anyway.
if you still want a video, just let me know.