Malcolm J. Rayfield wrote:
https://support.apple.com/HT202667
"If your email address ends with @icloud.com, @me.com, or @mac.com, you can't change your Apple ID to a third-party email address."
I'm getting the impression that the OP isn't talking about literally changing the Apple ID. When you Edit your Apple ID, you don't have "options" as to what other IDs you can use. I originally thought that is what they were trying to do as well, but now I'm not so sure....
GB