Remove original icloud email address
How do you remove your original iCloud email address which is now an alias act
Note the restrictions in the article.
Make sure you turn of Find My Phone before signing out of the old ID. Once signed out and Find My Phone is off, then sign in with the new ID, then turn Find My Phone back on.
What to do after you change your Apple ID or password - Apple Support * I suggest doing this before you change your Apple ID or password.