how do I set my @icloud.com as my apple id

I have had an Apple ID tied to a Gmail account for sometime now, but I have decided I would like to close the Gmail account and use my iCloud email instead. When I try to change my Apple ID from the Gmail address to the iCloud address I get a “try again later” error. When I tried changing it via Settings on my iPad, I got a message that said I could not change my Apple ID to an address already associated with my account. The iCloud address is an alias for my ID right now, so I suppose it is fairly same same, but I would like to remove the Gmail address from my Apple ID.

Thanks for the help!