Faction Change is now an available option in the Shop on Wrath of the Lich King Classic! There are bundle options, allowing players to faction change 3 characters and 8 characters at once.
Edit – It is now active on NA and EU Realms!
Edit – You can also purchase a Faction Change from the Battle.net Shop.
Faction Change Available in Shop Briefly – Credit Foolipe
One thing missing from the Shop is just same-faction Race Change, which players have been asking for since Burning Crusade.