Transfering data

Changing Apple ID email addresses is theoretically possible, and pragmatically difficult or impossible.

Whatever you’re trying to change to has already been used, either by you or another.

Here’s the description of the various email addresses, and the change-your-email-address sequence:

About your Apple ID email addresses - Apple Support

From that:

Change your Apple ID email address

To change the email address you use as your Apple ID, sign out of all Apple services and devices that use your Apple ID. Then follow these steps. 

There is no such thing as an “iCloud account”, there are Apple IDs. Those Apple IDs have associated iCloud storage, but they’re all Apple IDs.

Manually. For calendar entries, files, and related.

And app purchases and music and subscriptions and books won’t transfer.

How to transfer all files from one iCloud… - Apple Community