How to 'merge' different iCloud identities into one?

A good friend has been setting up an iCloud account using his 'Yahoo Email Address' to be the sign in identity. Then he got confused with the signing–in processes, associated passwords with on one side the Apple account, and on the other side the Yahoo–account.

We set up a new and logic iCloud identity for him—firstname.lastname@icloud.com

How is it possible to 'merge' all existing data from the old Apple account into the beautiful and correct one? Piece by piece i. e. photos export and import, address data export and import, etc. … else, or is there a professional offer by the great and smart Apple Support available?

Thanks a lot for your time and expertise!