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http://forums.worldofwarcraft.com/thread.html?topicId=20677331123&sid=1

QuoteThe new World of Warcraft Race Change service is now live. After purchasing a Race Change for a character, a player can choose any race of their same faction that is compatible with the character's current class. For more information on the Race Change service, please read the FAQ, or visit the Race Change page to get started.

Race Change FAQ: http://us.blizzard.com/support/article.xml?locale=en_US&tag=RCFAQ

Race Change Page: https://us.battle.net/login/login.xml?ref=https%3A%2F%2Fwww.worldofwarcraft.com%2Faccount%2Frace-change-intro.html&app=wam

So is this saying that as soon as Trolls are able to be druids we can change our tauren druids to Trolls?

From the Race Change FAQ Section:

Will I be able to select Cataclysm races and class combinations for a character once they're available in game?

Details are not available at this time. As development continues, though, we hope to provide more information.