# Connect to Exchange Online
$credential = Get-Credential
Connect-ExchangeOnline -Credential $credential

# Define the source and target domain
$sourceDomain = "olddomain.com"
$targetDomain = "newdomain.com"

# Get all users in the tenant
$users = Get-Mailbox -ResultSize Unlimited

# Iterate through each user and change the primary username
foreach ($user in $users) {
    $currentPrimary = $user.PrimarySmtpAddress.ToString()
    $newPrimary = $currentPrimary.Replace($sourceDomain, $targetDomain)
    
    if ($newPrimary -ne $currentPrimary) {
        Write-Host "Changing primary username for user: $($user.UserPrincipalName)"
        Set-Mailbox -Identity $user.UserPrincipalName -WindowsEmailAddress $newPrimary -EmailAddressPolicyEnabled $false
    }
}

# Disconnect from Exchange Online
Disconnect-ExchangeOnline