The Apple ID account servicing page makes provision for changing the email address associated with an account. Not just the "contactable at" address, but changing the primary address, using the option "Change Apple ID".
I want to do this for my five-year-old son's Apple ID – change it from the existing hisname@icloud.com to a Google Account hisname@gmail.com.
I have already added hisname@gmail.com against "Contactable At".
When I begin to change the primary address, I see the following pop-up, as expected (since I am the parent of the account-holder)…
The only problem is, that consent notice is never sent. Rather, the input form just updates to display the following error in red, including the bold HTML tags…
<b>Could not send request to Robert.</b><br/>Your Apple ID change request could not be sent at this time.
As a test, I have attempted to change the Apple ID address to test@mydomain.com (I can use any wildcard prefix at my domain). That time, the consent verification WAS successfully sent.
So, why won't Apple accept hisname@gmail.com?
Best Answer
From comments (& this took me years to figure out initially;)
You can't change an Apple ID address to one that is already in use as a 'Contactable at' or 'Recovery' address.
Once removed from there, then it will allow you.