Apple ID email address changes

Apple recently created ability to change the third party email address on Apple ID (e.g. gmail, yahoo) to an Apple email address (mac, iCloud, me).

The support page says:

If your Apple ID is a third-party email address

Before you change your Apple ID email address, you need to sign out of every Apple service and device that uses your Apple ID. If you want to use your iPhone, iPad, or iPod touch to change your Apple ID, stay signed in to your Apple ID on just that device.

When you change your Apple ID, you can enter another third-party email address. Or you can enter an @icloud.com, @me.com, or @mac.com email address that's already associated with your account. To see these addresses, check Reachable At in the Account section of appleid.apple.com.

https://support.apple.com/en-us/HT202667

Now, my questions:

1. If I have another Apple ID that is linked to said @icloud.com email address I wish to change the third party address to, what happens? Does it merge the 2 Apple ID's into one account?
2. If it does not merge accounts, what happens?
3. If it does merge accounts, has anybody experienced unintended consequences?

Thanks.