New Email address for AppleID

Ok.

What you want to do is CHANGE your AppleID using the instructions linked below.

Doing it any other way will NOT maintain everything in your current AppleID.

I worry that it might be problematic changing to an already existing Apple eMail. (as it usually defines a different AppleID)

Give it a try and post back if you encounter issues; we’ll proceed from there.

Change your Apple ID - Apple Support

An Apple ID is an email address. If you change the email address associated with your account, it will also change your Apple ID. However, since you say you use "Apple" (by which I presume you mean iCloud Mail) for email, you can use an alias instead. Your primary account ID for iCloud mail cannot be changed, but if you add an alias you can use that for email.

Add and manage email aliases for iCloud Mail on iCloud.com - Apple Support - "An email alias is an email address that conceals your actual email address from a recipient. An alias can help you manage the email you receive, and monitor the sources of unwanted messages. You can both send and receive email from an email alias using Mail on iCloud.com.

"With iCloud Mail, you can have up to three active email addresses, and you can turn an email alias on or off at any time. You can also delete email aliases that you no longer want to use. While turning off an email alias is temporary, deleting it is permanent. "

Is the new eMail address “Apple” or another eMail provider?