Dir *.png | rename-item -newname {  $_.name  -replace ".png",".pdg"  }
Dir *.pdg | rename-item -newname {  "000" + $_.name  }

//Batch Incremented File Rename
$files = Get-ChildItem c:\yourpath\*.ext
$id = 1
$files | foreach {
Rename-Item -Path $_.fullname -NewName (($id++).tostring() + $_.extension) 
}