```vba
Sub ConvertDocToDocx()
Dim strFolder As String, strFile As String, wdDoc As Document
' Set the folder and get the first .doc file
strFolder = "C:\Path\To\Your\Doc\Files\"
strFile = Dir(strFolder & "*.doc")

' Loop through each .doc file in the folder
Do While strFile <> ""
' Open the .doc file
Set wdDoc = Documents.Open(FileName:=strFolder & strFile)

' Save as .docx
wdDoc.SaveAs2 FileName:=strFolder & Replace(strFile, ".doc", ".docx"), FileFormat:=wdFormatXMLDocument

' Close the document
wdDoc.Close SaveChanges:=wdDoNotSaveChanges

' Get the next .doc file
strFile = Dir
Loop
End Sub
```

```powershell
$word = New-Object -ComObject Word.Application
$word.Visible = $false
Get-ChildItem 'C:\Path\To\Doc\Files\*.doc' | ForEach-Object {
$doc = $word.Documents.Open($_.FullName)
$doc.SaveAs([ref]$($_.FullName + "x"), [ref]16) # 16 is the wdFormatXMLDocument enum value for .docx
$doc.Close()
}
$word.Quit()
```