$ln = "AD\jdoe"
$nm = 'Doe, John'

foreach($s in (Get-SPSite -Limit ALL)){
    foreach($w in $s.AllWebs){
        $cu = $w.AllUsers[$ln]
        if($cu -ne $null){
             if($cu.name -ne $nm){
                 $w.Url
                 Write-Host "Replacing $($cu.name) with $($nm)"
                 $cu.DisplayName = $nm
                 $cu.Update()
             }
        }
    }
}