$Path = "C:\Temp\" 
$Word_App = New-Object -ComObject Word.Application

$Format = [Microsoft.Office.Interop.Word.WdSaveFormat]::wdFormatXMLDocument

$List = Get-ChildItem -Path $Path -Recurse -File -Filter '*.doc' | Where-Object { $_.Name -match '.+\.doc$' }
Foreach ($Item in $List) {
    if ($Item.Name -match '.+.doc$') {
        $Document = $Word_App.Documents.Open($Item.FullName)
        $Docx_Filename = "$($Item.DirectoryName)\$($Item.BaseName).docx"
        $Document.SaveAs([ref]$docx_filename, [ref]$Format)
        $Document.Close()
        #Remove-Item -Path $Item.FullName -Force
    }
}
$word_app.Quit()