Update UPN suffix for all users in preparation Azure AD Connect

Update-UPN.ps1

# Update the User Principal Name for all User Objects
$CustomDomain = "mcsmemail.de"

# Add custom domain as suffix first
Get-ADForest | Set-ADForest -UPNSuffixes @{add=$CustomDomain}

# Update all user objects
Get-ADUser -Filter * -Properties SamAccountName | ForEach-Object { Set-ADUser $_ -UserPrincipalName ($_.SamAccountName + "@$($CustomDomain)" )}