Change Apple ID

This article explains how this works.

Change your Apple ID - Apple Support

You can't change it to a third party email adress, Why? We don't know, we are not apple, we are users like you.

The relevant part for you is:

How to change your Apple ID if it ends with @icloud.com, @me.com or @mac.com

If you use an @icloud.com, @mac.com or @me.com email address as your Apple ID, you can’t change it to a third-party email address. But you can change it to an alias of your Apple ID address. For example, if j.appleseed@icloud.com is your Apple ID and you also have j.appleseed@me.com on file in your account, you can change your Apple ID to j.appleseed@me.com. When you change your email address, the Apple ID that people see when they share with you also changes.

To find an eligible email address that you can use as your Apple ID login, sign in to your Apple ID account page and click Edit under the Account section. You see Reachable At and a login alias with a question mark icon . If you hover over the icon, you see a message that says, “This is an alias of your Apple ID email address”.

You be able to easily change your Apple ID. See these instructions.

Change your Apple ID  https://support.apple.com/en-us/HT202667. Read these two sections very carefully to see what is allowed: If your Apple ID is a third-party email address and How to change your Apple ID if it ends with @icloud.com, @me.com, or @mac.com.

Before changing your Apple ID using a computer, sign out of all Apple services that use your Apple ID. If you are going to use an iPhone, iPad, or iPod, then stay signed in with your Apple ID on only that device. This instruction is taken from the above document under the heading Before you change your Apple ID.

For sign out purposes, see Where can I use my Apple ID? https://support.apple.com/en-us/HT202659 “This list is provided for informational purposes only and is subject to change without notice. It's not intended to include every feature or service that uses Apple ID. Visit the Apple ID support page for more information.”

And see What to do after you change your Apple ID or password 

 https://support.apple.com/en-us/HT204071

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But, you may want to create a new Apple ID https://support.apple.com/en-us/HT204316. See the data implications below.

Some of these things will apply even though not changing Apple ID: What to do after you change your Apple ID or password https://support.apple.com/en-us/HT204071

 Keep in mind that some things available under a first Apple ID will not be available under a new Apple ID. For example, you won’t be able to transfer anything purchased from the App and iTunes Stores to the new ID. And, there are other issues to be addressed.

And see what Eric Root says on this matter here https://discussions.apple.com/thread/251343949 and

what Winston Churchill says about it here: https://discussions.apple.com/thread/7452747