Apple ID won't accept my email address
I am trying to change my Apple ID from an @cikvn.com to an @gmail.com yet "Manage your Apple ID" page won't allow it: "cannot use this email address". Note: I have signed out of all Apple ID apps. Note, also, I had to do a systems upgrade to El Capitan yesterday and afterwards I was unable to sign-in to FaceTime, which is why I am trying to change my Apple ID. Help.