Changing Apple ID email address

From: Change your Apple ID - Apple Support

"If you see a message that the email address you want to use isn't available or is already in use, check that you or a family member aren't already using that email address with a different Apple ID. If a family member is using that email address, remove it from their account then try to use it."

Although this next section discusses email address re-use in the context of deleting an account, it supports observations frequently posted on this forum about not being able to re-use an email address as an Apple ID once used previously as an Apple ID or even as a notification or rescue email address:

From: How to delete your Apple ID account - Apple Support

"Can I use the primary email address for my deleted Apple ID account to create a new Apple ID or with another existing Apple ID account?

No, once your Apple ID is deleted, you can't use the email address that you used with it to create a new Apple ID. In addition, you can't use that email address with another existing Apple ID except as a rescue [notification*] email. Also, if your primary Apple ID email address is with a Custom Email Domain, it can never be reused to create an Apple ID after your Apple ID is deleted, even if the domain ownership changes."

* About your Apple ID email addresses - Apple Support

If you still have questions or problems:

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