Alias in Apple ID.
I can't seem to change the email Alias for my apple ID account. The old email is no longer vaild and I want to update it.
MacBook Pro 13", macOS 10.15
The alias or the email address associated with your Apple ID?
You can't remove/change @iCloud.com, @me.com or @mac.com aliases.
iCloud - Create or change email aliases
Note the restrictions in the article.
What to do after you change your Apple ID or password - Apple Support I suggest doing this before you change your Apple ID or password.