.NET Convert .RTF to .DOTX

not working

Im doing this:
It makes DOCX only, i need dotx

DIM dotXLoc as string = "c:/temp/word.docx"
DIM rtfLoc as string = "c:/test/test.rtf"

Dim wordApp As Object = New Microsoft.Office.Interop.Word.Application()
Dim currentDoc As Microsoft.Office.Interop.Word.Document = wordApp.Documents.Open(rtfLoc)
currentDoc.SaveAs(dotXLoc, Microsoft.Office.Interop.Word.WdSaveFormat.wdFormatDocumentDefault)
currentDoc.Close()
wordApp.Quit()

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If i change to DOTX extension  
dotXLoc  = "c:/temp/word.DOTX"
, it still saves in docx format, so basically, to open the file, i have to manually change the ext to docx in windows explorer

Thanks