Microsoft 365 – Changing default username in Azure AD Connect synced environment

Issue: If a user was imported via AD but shows up with the wrong username on the O365 side (ending in .onmicrosoft.com)

Resolution:

1. Go to Active Directory Users and Computers
2. Go to View and select Advanced Features
3. Go to properties of user account
4. Go to Attribute Editor
5. Edit Proxy Address to: SMTP:user@domain.com
6. Open PowerShell as Admin
7. Connect to the online service with the following commands: (If needed Install-Module MSOnline)
   1. $msolcred = get-credential
   2. Connect-MsolService -Credential $MSolCred
      (Note: Enter your Office 365 credentials when prompted)
8. Enter the command to update the user name:
   1. Set-MsolUserPrincipalName -UserPrincipalName user@company.onmicrosoft.com -NewUserPrincipalName user@domain.com

Changes should take effect within a few moments and set this user as the default username.