Please allow us to rename existing @icloud.com Apple IDs to a mail aliases defined on the same account (that's the additional email address defined via the Preferences pane of the Mail app on icloud.com).
An Apple account is identified by its Apple ID, which for a long standing account, has a lot of information associated with it. However, an account handle set up years ago can become irrelevant over time (people get married, change their names, etc).
We already can define Mail aliases for existing Apple IDs. These aliases are used with Mail, Messages and Facetime... but not when we share content via the share button/menu (ie: if you share a note or a reminder with someone the content will be shared using your Apple ID).
Currently, the only way to fix this is to create a brand new Apple ID, and then migrate all of your existing content to that Apple ID. However, this is not an acceptable solution to many people.
Wouldn't it be great if we could change our old *@icloud.com Apple ID to one of the mail aliases setup on the account? I imagine there is a plethora of Apple Users who would benefit immensely from being able to evolve their accounts with their changing live circumstances.
Unfortunately, nothing is likely to change until we raise Apple's awareness of this issue.
If you're affected by this, or if you would like to see this implemented, please visit iCloud Feedback page (use Feedback Type: Account) and submit the feedback form to let Apple know that you too would like this feature implemented.
PS: Apple IDs can already be changed if you use a 3rd party email address or your Apple ID ends in @me.com or @mac.com (https://support.apple.com/en-us/HT202667#appleaddress). However, Apple IDs that end in @icloud.com are for some reason excluded.