"You can now buy account upgrade keys and download the game client for World of Warcraft: The Burning Crusade directly through the account management section of Worldofwarcraft.com," the company said.
"With this new direct online upgrade method, you don't even need to leave your desk if you decide to upgrade to The Burning Crusade."
Making the game downloadable is expected to enhance its already gigantic success.
In January, the first expansion pack for World of Warcraft became the fastest selling PC game of all time.
According to statistics released by Blizzard, the Burning Crusade add-on sold over 2.4 million copies on its first day.
US sales totalled 1.2 million copies while 1.1 million boxes were sold in Europe on the first day.
By the end of the day, 1.7 million of those eager buyers had logged in and upgraded their version of Warcraft. In its first week, sales have topped three million copies.
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