@jose-0 Hiya,
I'm not sure I totally understand your use case, but this should be doable using the APIs and some code. But it isn't going to be available out of the box.
If I were to do this, I'd build a page which let a user supply two email addresses and specify which one should be primary.
Then I'd write some code to send a random number to each email address and have them enter the number on that same page.
If the codes were correctly entered, I'd presume that the user was the same (all they really proved is that they had access to each of the email inboxes, though. If you wanted more assurance, you could use step up auth). Then you can use the User API to:
update the primary account with any data that should be moved. I'd definitely move the secondary email (perhaps in an array on the user.data field). They won't be able to use this email address to log in any more, but it still might be useful for your support staff or to display in your UX
lock the secondary account so it is not usable any more.
None of this is out of the box with FusionAuth, but building on the APIs it should absolutely be possible.
Hope that helps, thanks for using FusionAuth!