This script run word application silently to convert ".doc" files to ".docx" files, include to not show the SaveAs dialog window. It's work recursively to convert all availables files.

Convert-Doc-To-Docx.ps1

<#
.SYNOPSIS
  Convert ".doc" files to ".docx" files recursively.
.DESCRIPTION
  This script run word application silently to convert ".doc" files to ".docx" files, include to not show the SaveAs dialog window.
  It's work recursively to convert all availables files.
.EXAMPLE
  .\Convert-Doc-To-Docx.ps1 -FolderPath 'c:\FolderWithDocFiles\'
#>

Param(
    [Parameter(Mandatory=$true)][string]$FolderPath=$(throw "You haven't set a folder path.")
)

[ref]$SaveFormat = “microsoft.office.interop.word.WdSaveFormat” -as [type]
$word = New-Object -ComObject word.application
$word.visible = $false
$fileType = “*doc”

Get-ChildItem -path $folderpath -include $fileType -Recurse |
foreach-object `
{
    $path = ($_.fullname).substring(0,($_.FullName).lastindexOf(“.”))
    “$path”
    $doc = $word.documents.OpenNoRepairDialog($_.fullname)
    $doc.saveas(([ref] $path).Value, ([ref]$SaveFormat::wdFormatDocumentDefault).Value)
    $doc.close()
    rm -Force -Recurse "$path.doc"
}

$word.Quit()
$word = $null
[gc]::collect()
[gc]::WaitForPendingFinalizers()