Probably this is the easiest and very reliable option to convert the xls to xlsx conversion.
“C:\Program Files (x86)\Microsoft Office\Office15\excelcnv.exe” -oice “C:\in\MyFile.xls” "C:\out\MyFile.xlsx”
https://gist.github.com/Opiprog/8bb6f491578d4f23540ccd98e85288c3