If you wanna downgrade world of warcraft from latest 3.0.3 to an earlier 3.0.1 or 3.0.2 simply go to C:\....\World of Warcraft\Data and delete patch.mpq. After that run the repair.exe and check files. When that's done your version is now 3.0.1. If you want 3.0.2 just update from here.
Hope this helps someone out there