Converting XLS to XLSX File

Hi

I'm trying to save XLS file as XLSX file using below code.
XlApp DotNet Microsoft.Office.Interop.Excel.ApplicationClass.'Microsoft.Office.Interop.Excel, Version=15.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c' 
XlWrkBk DotNet Microsoft.Office.Interop.Excel.Workbook.'Microsoft.Office.Interop.Excel, Version=15.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c' 
XlWrkSht DotNet Microsoft.Office.Interop.Excel.Worksheet.'Microsoft.Office.Interop.Excel, Version=15.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c' 
XlHelper DotNet Microsoft.Dynamics.Nav.Integration.Office.Excel.ExcelHelper.'Microsoft.Dynamics.Nav.Integration.Office, Version=10.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35'  
ExcelXlSaveAsAccessMode DotNet Microsoft.Office.Interop.Excel.XlSaveAsAccessMode.'Microsoft.Office.Interop.Excel, Version=15.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c' 
ExcelFileFormat DotNet Microsoft.Office.Interop.Excel.XlFileFormat.'Microsoft.Office.Interop.Excel, Version=15.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c' 

XlsFilename := C:\123.xls;
XlsxFilename := C:\123.xlsx;
XlApp := XlApp.ApplicationClass;
XlHelper.CallOpen(XlApp,XlsFilename);
XlWrkBk := XlApp.ActiveWorkbook;

XlWrkBk.SaveAs(XlsxFilename,ExcelFileFormat.xlWorkbookDefault,'','', FALSE, FALSE,
  ExcelXlSaveAsAccessMode.xlNoChange, FALSE, FALSE, FALSE, FALSE, FALSE);
XlWrkBk.Close(FALSE,'',TRUE);

but receiving the below error.

Type mismatch. (Exception from HRESULT: 0x80020005 (DISP_E_TYPEMISMATCH))