If you made the mistake of downloading the WoTLK client with WoD models and are suffering from crashes, spooky players with no faces, female humans with no hair, etc., you don't really need to download the old client from scratch.
To upgrade from the buggy WoD client to the well working old client:
- Go to the folder you downloaded the WoW client to using Explorer/Finder (Windows/OS X).
- Open the Data folder.
- Delete the patch-w.mpq file.
- Open the enUS folder.
- Delete the patch-enUS-w.mpq file.
That's it.