Change Modified by

Hi 

Just migrated from one Domain to another - and my user account have edited some site pages, so I now are seen in the 'Modified By' - I would like to change this to the System account/Farm Account 

I found this Script - but need Help to change this to only the 'Modified by' - not touching the 'Created By': 

[Reflection.Assembly]::Load("Microsoft.SharePoint, Version=12.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c")
$SPSite = New-Object Microsoft.SharePoint.SPSite("http://dev/")

$SPWeb = $SPSite.OpenWeb()

$SPList = $SPWeb.Lists["ModifyCreatedBy"]

$SPListItemCollection = $SPList.Items

foreach ($ListItem in $SPListItemCollection) 
    {
       $SPFieldUserValue = New-Object Microsoft.SharePoint.SPFieldUserValue($SPWeb,8, "contoso\myuser")
        $ListItem["Author"] = $SPFieldUserValue                   
        $ListItem.Update() 
    }
    $SPWeb.Update()