Update UPN and add Proxy Address Attribute's - Sources: https://www.reddit.com/r/PowerShell/comments/f001c8/updating_proxyaddresses_attribute_help/?utm_source=share&utm_medium=ios_app&utm_name=iossmf | https://pastebin.com/PGmMphdQ

Update-DomainUPNandProxy.ps1

$oldDom = '@newdomain.com'
$newDom = '@olddomain.com'

# USING GET-ADUSER AS INPUT:
# Get all AD users with an email address that ends in the old domain.  These will be the ones to change.
$adFilter = 'mail -like "*{0}"' -f $oldDom
$users = @(Get-ADUser -Filter $adFilter -Properties mail, proxyAddresses)

# Loop and process each found user
foreach ($user in $users)
{
    # Locate old primary proxyAddress
    $oldPrimary = $user.proxyAddresses.Where({$_.StartsWith('SMTP:')})

    if ($oldPrimary.Count -ne 1)    # That's weird...
    {
        Write-Warning ("{0} somehow either has multiple or no primary SMTP addresses?" -f $user.Name)
        continue
    }
    else
    {
        $oldPrimary = $oldPrimary | Select-Object -First 1
    }

    # Format the new 'mail' property for the given user.
    $replacedEmail = $user.mail.Replace($oldDom, $newDom)

    # Use the replaced value to make the new primary proxyAddress.
    $newPrimary = "SMTP:{0}" -f $replacedEmail

    # Format the new UPN
    $newUpn = $user.UserPrincipalName.Replace($oldDom, $newDom)

    # Transform the 'old' primary from "SMTP:" to "smtp:".
    $oldAlias = "{0}{1}" -f $oldPrimary.Substring(0, 4).ToLower(), $oldPrimary.Substring(4)

    # Announce
    Write-Host ("Will replace '{0}' with '{1}'" -f $oldPrimary, $newPrimary) -f Yellow
    Write-Host ("The old alias will be: {0}" -f $oldAlias) -f Yellow
    Write-Host ("The new UPN will be: {0}" -f $newUpn) -f Yellow

    # Create new collection containing all current proxyAddresses
    $addressCol = [Microsoft.ActiveDirectory.Management.ADPropertyValueCollection]::new($user.proxyAddresses)

    # Find the index within the new collection of the old primary address.
    $index = $addressCol.IndexOf($oldPrimary)

    # Replace that entry with the 'new' primary address.
    $addressCol[$index] = $newPrimary

    # Then add the old primary as an additional proxyAddresses (alias)
    [void] $addressCol.Add($oldAlias)

    # Build 'Set-ADUser' arguments
    $setArguments = @{
        Identity = $user.DistinguishedName
        EmailAddress = $replacedEmail
        Replace = @{ proxyAddresses = $addressCol.Value }
        UserPrincipalName = $newUpn
		WhatIf = $true      # Get rid of me to perform the changes
    }

    # Execute
    Set-ADUser @setArguments

    Write-Host "--------------------" -f Yellow     # Just a separator between each loop.
}