Changing Models

Ok, i would like to know if it is possible to Change the animal that the farseer rides from a Wolf to a Wyrven.

You would need to do model editing. The old Alpha model exists, (I imagine that's what you're talking about), but only in Blizzard's archives. And they won't let anyone else get to it.

which is why i need to modify it. So how would i do this?

Very tricky. Go to the modelling forum and seek out Cookie.