How to change the texture of a model, with procedures?

I don't think you can chance the model with procedures, what I have been doing is making a copy of the mob with the different texture and model, and then swapping them. 

how do you swap the mob? if you have time to answer of course

To swap a mob for another with a different model your first have to make both mobs. Then you go to the trigger section of the first mob and select the one you want to trigger the change (say, "when entity is hurt") and there you have to make a procedure with the blocks "despawn target entity", and then "spawn at x y z living entity" being the second entity. You can add a few more things, but that's the basic. 

It's easy to implement through editing entity source code, but only if you are familiar with java