Concerning WoD models on a MoP server

I understand that you need to have a 5.4.8 client. Why exactly do I need to re-download the entire game again? My internet speed isn't great so I don't really want to spend a week downloading a game I already have just so I can enjoy the models that were working perfectly fine before the update to 5.4.8.

Why exactly doesn't the 5.4.2 "updated" client work? It is really frustrating that I already have all this information and data on my computer, but it doesn't just work? What's the reason for updating to 5.4.8? Basically I just want to know why I can't perform a patch or download SOME of the files from the 5.4.8 client/game/whatever