How can I change my Apple ID to @iCloud.com?

I followed all instructions posted on iCloud support, but none of them answered correctly.

I'm also following these steps...

1. Go to Settings > iCloud > Account, then tap Mail under Advanced.
2. Under iCloud Account Information, tap your email address.
3. From the list of addresses available, tap the @icloud.com email address that you want to use as the default address for sending mail from iCloud Mail.

...but still can't change my iCloud addres.