I’m a little surprised at how confusing this change seems to be for some. Here’s what the deal is in simple:
You created an Apple ID using a third party email address. At some point later you joined iCloud. This happened for many who may have only used an Apple ID before iCloud was rolled out and who also didn’t use iTools/MobileMe. So when iCloud was rolled out and you joined in, you created an iCloud email address that was tied to your Apple ID. So basically this change is to let you change your Apple ID from the third party email to the iCloud email address you created already. It may allow for a couple other scenarios but I think the intention is that if you have, for example, and Apple ID of myaddr@gmail.com and an iCloud email of otheraddr@icloud.com then you can change your AppleID from myaddr@gmail.com to otheraddr@icloud.com. If you’re wanting to change otheraddr@icloud.com you probably would need to do it as a later step.
OK, that is exactly what appleid site "agrees" to do for me (I didn't confirm the change though)
I spent like an hour, trying to be creative
After reading some guesses here, I tried one of my aliases I have created during playing initially with iCloud services long time ago. .... drum roll.... all of my existing aliases on my apple id account are welcome to be my primary id (by switching from my original 3rd party email address)
So, I guess it is working.... Will try again and complete the process from home.
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yup - looks like change to alias at .icloud, .me, .mac works fine.