Hi,
When a user renames an account, the old account name will be the ALIAS of the new one. Please be informed that if an account becomes an alias, you cannot use it as sign in address. It will only be used for sending and receiving emails of the primary account (which is your outlook.com account). You will continue receiving the emails sent to the old address by either redirecting it to an alias folder or to your Outlook.com inbox.
Example:
******@live.com was renamed to ******@live.com
******@live.com will become the alias of ******@live.com.
In this case the user won't be able to log using their old account (******@live.com).
For instructions on how to rename the account, please refer to this article.
Please refer to the article below for additional details:
What are some things to consider when renaming to an @Outlook.com account?
Thanks