Changing Apple ID from old Yahoo email to iCloud.com

You can only use a primary iCloud email address for an Apple ID, not an iCloud mail alias.

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."

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."

Here's how you change the email address (Apple ID) you use for your Apple Account while retaining everything in your account:

1. Go to https://appleid.apple.com and sign in.

2. In the Sign-In and Security section, choose Apple ID.

3. Enter the email address that you want to use as your Apple ID.

4. Choose Change Apple ID.

5. If you changed your Apple ID to a third-party email address, check your old ID email for a verification code, then enter the code. Learn what to do if you didn't get the email. --> If you didn't receive your verification or reset email - Apple Support

6. If you use services like iCloud or Messages to share with friends and colleagues, sign in to those services with your updated Apple ID. --> What to do after you change your Apple ID or password - Apple Support

For more details, read the document: "Change your Apple ID" --> Change your Apple ID - Apple Support

You cannot change a primary iCloud mail address once set up. You would have to create a new Apple ID but then there would be the immense nuisance of transferring things over, plus some things (notably purchases) cannot be transferred between accounts.

If you have created an iCloud mail account for your Apple ID you should be able to replace the third party address with the iCloud address after a 30 day wait period. Personally I would not use an iCloud address as my Apple ID because should you have problems with your account then you also lose the address to which Apple is sending things to help you with your account.

Refer to this document for ways to contact Apple ➞ Choose your country or region - Official Apple Support

Select your country (also look for "other" regions), then a product. If you don't see one that handles your issue then keep experimenting with selections until you reach one that gets you a chat session or a telephone call and get the representative to redirect you.

or:

Contact Apple for support and service by telephone ➞ Contact Apple for support and service - Apple Support