How do I change my iCloud account but have the same Apple ID?
How do I change my iCloud account and have all that data too from the previous iCloud account into the new one all in the same Apple ID
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You can change your Apple ID email address on your current Apple ID so you retain access to your data and prior purchases. Note the restrictions in the article.
Make sure you turn off 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.