Apple ID change

I believe you cannot change from an email address to a phone number, only to a different one of the same kind.

Use your mobile phone number as your Apple ID - Apple Support

talks about changing one phone number to a different one, plus about regional limited availability of using a mobile number but only when you are creating a new Apple account.

Change your Apple ID - Apple Support

talks about changing from an email address to an email address.

On a related theme: Now my Apple ID my mobile no I want chang… - Apple Community

August 2021 post by a Community Specialist about changing from a telephone number Apple ID to an email format:

"While it's possible to change your Apple ID to a different phone number, there isn't a way to change it to an email address."

That's related to what was mentioned in a previous post, "Change your Apple ID to a different mobile phone number"

Note it says to a "different" number, i.e., changing from a mobile number ID to another mobile number. You are wanting to change from an email address to a number.

From: Use your mobile phone number as your Apple ID - Apple Support

Change your Apple ID to a different mobile phone number

Depending on the country or region that you live in and how you created your Apple ID, you might be able to change your Apple ID to a different mobile phone number from your Apple ID account page or on your iPhone, iPad, or iPod touch.

On the web:

1. Sign out of every Apple service and device that uses your Apple ID.
2. Go to appleid.apple.com and sign in.
3. In the Sign-In and Security section, click Apple ID.
4. Enter the mobile phone number that you want to use as your Apple ID, then select Change Apple ID. 
5. A verification code is sent to you at that number. Enter the verification code from the text into the fields provided.
6. Sign back in to all Apple services with your new Apple ID.