Hi Eric,
I think you are right about the root of the problem: Apple doesn't allow you to re-use a notification email address as an Apple ID. It also doesn't allow re-use of a previously-used email address. So if you change your mind about how you prefer to be contacted, you are out of luck.
Also: there appears to be no provision for consolidating duplicate Apple IDs.
I had at some point rather recently made a purchase in store (I think it was an iPhone) and they set up a new Apple ID for me at that time using my current preferred email. But I already had an older Apple ID (@yahoo account) email which I no longer used, but was associated with my previous Apple purchases. This older Apple ID had my purchase history, family sharing and credit card preferences saved.
Once I found there were two accounts, I simply wanted to change the old account to the newer email, but was unable to do so because Apple has no way of saying "We noticed you have two accounts. Would you like to combine them?" And they don't allow the use of a previously-used email.
The whole process was made more difficult because they don't just come out and say "There's already an Apple ID associated with that email". Instead you just get a vague error message. So I didn't understand why it wasn't working initially.
I'm not sure why they can't just alert you to the problem, authenticate your identity using challenge questions and 2-factor authentication, and then allow you to consolidate your accounts to contact you however you prefer. Especially when creating a new account is so easy. I don't understand the penalty for using a previously-used address. I'm surely not the first person to change their primary email address. I also didn't understand the consequences of setting up and Apple ID in-store when making that purchase.
I ended up creating a new, Apple-specific Gmail account as a workaround. It's an all-new address, but it defeats the whole purpose of trying to change my preferred email address in the first place: to have any important messages from Apple delivered to my primary email inbox - and to avoid them going to an address I no longer use.
