Managing personal Outlook.com account settings, security, and privacy
Hi Raven,
Since all four @outlook.com addresses already appear as aliases on the same Microsoft account, you should normally be able to select “Make primary” beside any one of them. Microsoft confirms that this is the supported way to change the primary alias.
Since you have been getting the same error for over a year with every alias, this is unlikely to be a temporary browser issue. I would contact Microsoft Account Support and ask them to investigate why the account is refusing the primary-alias change.
One important warning: do not remove your current @hotmail.com alias while troubleshooting. Microsoft states that removing a Microsoft-domain alias permanently deletes that address, and it cannot be added back later.
Thank you, and feel free to respond back for more assistance.