Apple makes it easier to change your registered email
 

Does it irk you that you can't replace your Gmail, Outlook or any other third-party Apple ID with one of your Apple email addresses? Sure, it's a minor inconvenience, but if you really prefer using an @icloud.com, @me.com or @mac.com email account to...

https://www.engadget.com/2017/11/01/...-email-change/