Change Primary Apple ID to an Existing …
https://discussions.apple.com/thread/7809393
Go to http://appleid.apple.comand click 'Manage your Apple ID'. Sign in with the current ID. Where it says 'Apple ID and primary email address' and gives your current ID email address, click 'edit'. Enter your new address and click 'Save changes' Sign back into iCloud on all your devices with the new ID and password. Log into your old Apple ID at appleid.apple.com. At the Account section at the top, click the Edit button at right. Under Apple ID, click Change Apple ID. Enter a new email address that’s something else, at least temporarily. This can’t be registered with another Apple ID. Follow the confirmation and verifications. Log into your old Apple ID at appleid.apple.com. At the Account section at the top, click the Edit button at right. Under Apple ID, click Change Apple ID.
Log into your old Apple ID at appleid.apple.com.
At the Account section at the top, click the Edit button at right.
Under Apple ID, click Change Apple ID.
Enter a new email address that’s something else, at least temporarily. This can’t be registered with another Apple ID.
Follow the confirmation and verifications.
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