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On iPad or iPhone I image you can switch off contact syncing, choose to leave the contacts on the device, log out of the me.com account and then into the new one. This should merge the contacts, if you don't have any in the new account it will definitely copy them.
 
Sync the iPhone with the old MobileMe account then remove the old account from the phone. When prompted, keep the contacts on the phone and add the new MobileMe account to the phone.
 
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