How can I change my primary Apple ID account?

An Apple ID account is for life. You can change the email address used to access the account, but if you get a new Apple ID account that all purchases made with the old Apple ID must be made all over again, and cannot be transferred to the new Apple ID. And also if an Apple device is activation locked to the old Apple ID you will no longer be about to reset or sell the the phone, because it will require the old Apple ID and password to remove activation lock.

You can only disable Find my iPhone with the Apple ID that the phone was logged in with when Find My was enabled. There is no other way to defeat this theft deterrent feature.