When using Azure AD Connect, the userPrincipalName attribute in Active Directory is used to set the user name in Office 365. This username is used for all Office 365 services such as Outlook Web Access, ActiveSync, SharePoint, etc.
It’s not possible to change the username in Office 365 admin console and you will receive a message, “This user is synchronized with your local Active Directory. Some details can be edited only through your local Active Directory.”
When changing the userPrincipalName attribute in Active Directory it still won’t synchronise the change to Office 365, even when using the -PolicyType Initial flag. The only way to update the username / User Principal Name in Office 365 is to use the Microsoft Azure Active Directory Module for Windows PowerShell with the the following command:
Set-MsolUserPrincipalName -UserPrincipalName finance@contoso.onmicrosoft.com -NewUserPrincipalName finance@contoso.com
To create an Office 365 mailbox for an existing user, the command we’ll be using is enable-remotemailbox.
enable-remotemailbox -Identity "New User Account" -RemoteRoutingAddress new.user@contoso.mail.onmicrosoft.com
After complete, go to 365 admin center and assign a license.