Please allow users to change their current @icloud.com AppleID to an @icloud.com mail alias, that's the additional email address that can be defined via the Preferences pane of iCloud's Mail web app.
An Apple account is identified by its AppleID, which for a long standing account, has a lot of information associated with it. However, people grow up, get married, change their names. Not everyone wants to set up a new Apple ID when that happens. However, an account handle set up years ago can become irrelevant over time.
Apple IDs can already be changed if you use a 3rd party email address or have a mail alias that ends in @me.com or @mac.com (https://support.apple.com/en-us/HT202667#appleaddress). However, mail aliases that end in @icloud.com are for some reason excluded.
It would be great if users who's Apple ID already ends in @icloud.com could set one of their @icloud.com mail aliases as their new AppleID. I imagine there is a plethora of Apple Users who would benefit immensely from bei