To rename a user account (including email addresses), do the following:
- Right-click the user account and select Rename from the context menu.
- Rename all relevant name attributes and click OK.
- Open user properties and select the Mailbox tab.
- Rename the Alias and click Apply.
- Select the Office 365 tab and click Proxy Addresses.
- Click Create from domains and configure the new primary SMTP address.
- Click OK, select Synchronize AD with Office 365, and click OK.