Thats what I tried. It gives a message as:
"You cannot change this Apple ID to use an existing Mobileme email address. The email address you entered is already associated with an Apple ID. If its yours you can cancel and signin using that Apple ID."
So, how to go about this. If I login using icloud id, then it doesn't transfer my existing downloads to icloud id. So I want to update the email id associated with the Apple account.