Changing Apple id

Hi

Go to appleid.apple.com and sign in.

In the Account section, choose Edit.

Choose Change Apple ID.

Enter the email address you want to use. 

If you are using a third party email address check your emails for a verification code.

If You use other Apple services will need to sign using your new Apple ID

Try setting up a new notification/rescue email address and then try changing the ID. This is likely similar to what Apple says in this document:

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."