I looked into this problem sometime ago, and wasn't able to find a solution. Other than this article which didn't work.
Apparently, you need an xbox360 and you can't do it on the new xboxone/x if it's even possible at all. Microsoft Support at the time was absolutely useless. Kept insisting that I just wanted to change the email address.
So, I'm hoping you guys might know more than I.
Anyway, my question is this:
Old Microsoft Account and old email address with Xbox Account
Transfer to the new:
Microsoft Account and email
So I can delete the old email address and account because I havn't used it in over 5 years.
Thanks for any help in advance.