The change to Outlook.com in April provided the ability to sign in with any alias that has been added to your account as well as simplifying the process of adding and managing aliases.
With the change today, "Rename" is being replaced with the ability to make any of your aliases a primary alias. Because it is possible to sign in with any alias, there is little need to use the option to change your primary alias, unless you want a different email name to show on your Microsoft devices.
You can set up or manage aliases at https://account.live.com/names/Manage.
If you are confused between alias accounts and the old linked accounts, see Managing Formerly Linked Microsoft Accounts.
- A better way to manage aliases and primary email address for your Microsoft account
- Use aliases to add email addresses to your account