# 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 groups in the tenant
$groups = Get-UnifiedGroup -ResultSize Unlimited

# Iterate through each group and change the email address
foreach ($group in $groups) {
    $currentPrimary = $group.PrimarySmtpAddress.ToString()
    $newPrimary = $currentPrimary.Replace($sourceDomain, $targetDomain)
    
    if ($newPrimary -ne $currentPrimary) {
        Write-Host "Changing email address for group: $($group.DisplayName)"
        
        # Change the email address for the group
        Set-UnifiedGroup -Identity $group.Identity -PrimarySmtpAddress $newPrimary
        
        # Change the email address for the group's underlying mailbox (if applicable)
        if ($group.RecipientTypeDetails -eq "GroupMailbox") {
            Set-Mailbox -Identity $group.PrimarySmtpAddress -WindowsEmailAddress $newPrimary
        }
        
        # Change the email address for any associated distribution lists (if applicable)
        $distributionLists = Get-DistributionGroup -RecipientTypeDetails MailUniversalDistributionGroup, MailUniversalSecurityGroup -LinkedGroup $group.Identity -ResultSize Unlimited
        foreach ($distributionList in $distributionLists) {
            Set-DistributionGroup -Identity $distributionList.Identity -PrimarySmtpAddress $newPrimary
        }
    }
}

# Disconnect from Exchange Online
Disconnect-ExchangeOnline