Change display name for existing users

Question

Hi,
We have alot of users in our Active Directory. After an migration our existing users are displaying their name wrong. When we create a new user the display name is set FirstName LastName. But all of our old users have their User logon name as display name.
Is there a way to change the display name of our old users to FirstName LastName instead of the user logon names they are displaying now? Doing this per user is a hell of a job so this really isn't an option.
Example:
Full name: Peter Peterson. User logon name: pepeter. Display name: pepeter. (old user)
Full name: Tom Thompson. User logon name: tothomp. Display name: Tom Thompson. (new user)
Greetz,
Tom

Answer 1

I have found a script for Powershell that may be the answer to my question. There is only one thing that is not in the script. The script runs for all users. The PageSize is set to 2000 so it runs 2000 entries. Is there a way that I can run this script for
all Users in AD that start with the letter "A" or with an other letter. :P
$previewMode = $true #change value to $false ONLY after you have verified the preview!!!
$CurrentDomain = [System.DirectoryServices.ActiveDirectory.Domain]::GetCurrentDomain()
$Domain = [System.DirectoryServices.DirectoryEntry]"LDAP://$CurrentDomain"
$Searcher = New-Object System.DirectoryServices.DirectorySearcher
$Searcher.PageSize = 2000
$Searcher.SearchScope = "Subtree"
$Searcher.Filter = "(&(objectCategory=person)(objectClass=user)(!(objectClass=computer)))"
$Searcher.PropertiesToLoad.Add("userPrincipalName") > $Null
$Searcher.PropertiesToLoad.Add("givenName") > $Null
$Searcher.PropertiesToLoad.Add("sn") > $Null
$Searcher.PropertiesToLoad.Add("distinguishedName") > $Null
$Searcher.SearchRoot = "LDAP://" + $Domain.distinguishedName
$SearchResults = $Searcher.FindAll()
$dnsSuffix = $CurrentDomain.Name
foreach ($Result in $SearchResults){
               $firstName = $Result.Properties.Item("givenName")
               $lastName = $Result.Properties.Item("sn")
               $newLogonName = "$firstName $lastName"
                [System.DirectoryServices.DirectoryEntry]$userObject = $Result.GetDirectoryEntry()
                $oldLogonName = $userObject.sAMAccountName
    Write-Host "Changing $oldLogonName to $newLogonName..."
    try {
        if (!($previewMode)) {
                   $userObject.cn = $newLogonName           
           $userObject.CommitChanges()
        Write-Host "$oldLogonName successfully changed to $newLogonName."
    catch{
        Write-Host "Error changing $oldLogonName to $newLogonName."