With the 64-bit client files in place you may receive errors such as : Blizzard Updater failed with an internal error. or the Play button may stay Grey on the Launcher.
Simply delete the 64-bit files listed below from the World of Warcraft folder, and the patching process will resume after going through a short repair process.
World of Warcraft-64.app
A new version of the 64-bit client files will be required as well. Please keep an eye on the 64-bit client forum thread for updated files: