From:
Change your Apple ID - Apple Support
- There's special instructions if you want to change your Apple ID to an @icloud.com address.
"If you try to change your Apple ID to an @icloud.com email address that was created within the last 30 days, you will be asked to try again later."
And a related article about using iCloud email aliases - Add and manage email aliases for iCloud Mail on iCloud.com - Apple Support - "Note: You can’t delete or turn off a primary iCloud Mail address. You can’t use an alias to create a separate Apple ID, and an email alias can’t be converted to a primary iCloud Mail address."