private string ConvertToXSLX(string path)
{
    using (var fs = File.OpenRead(path))
    {
        var result = XLS_to_XLSX_Converter.Convert(fs);

        // (file.xls) + x = file.xlsx
        path = $"{path}x";

        using (var fs2 = new FileStream(path, FileMode.OpenOrCreate, FileAccess.Write))
        {
            fs2.Write(result, 0, result.Length);
        }
    }
    return path;
}

public static byte[] Convert(Stream sourceStream)
{
    var source = new HSSFWorkbook(sourceStream);
    var destination = new XSSFWorkbook();

    for (int i = 0; i < source.NumberOfSheets; i++)
    {
        var hssfSheet = (HSSFSheet)source.GetSheetAt(i);

        var xssfSheet = (XSSFSheet)destination.CreateSheet(source.GetSheetAt(i).SheetName);

        CopyRows(hssfSheet, xssfSheet);
    }

    using (var ms = new MemoryStream())
    {
        destination.Write(ms);
        return ms.ToArray();
    }
}

private static void CopyRows(HSSFSheet hssfSheet, XSSFSheet destination)
{
    for (int i = 0; i < hssfSheet.PhysicalNumberOfRows; i++)
    {
        destination.CreateRow(i);

        var cells = hssfSheet.GetRow(i)?.Cells ?? new List<ICell>();

        for (int j = 0; j < cells.Count; j++)
        {
            var row = destination.GetRow(i);
            row.CreateCell(j);

            CopyCell((HSSFCell)cells[j], (XSSFCell)row.Cells[j]);
        }
    }
}

private static void CopyCell(HSSFCell oldCell, XSSFCell newCell)
{
    CopyCellValue(oldCell, newCell);
}

private static void CopyCellValue(HSSFCell oldCell, XSSFCell newCell)
{
    switch (oldCell.CellType)
    {
        case CellType.String:
            newCell.SetCellValue(oldCell.StringCellValue);
            break;
        case CellType.Numeric:
            newCell.SetCellValue(oldCell.NumericCellValue);
            break;
        case CellType.Blank:
            newCell.SetCellType(CellType.Blank);
            break;
        case CellType.Boolean:
            newCell.SetCellValue(oldCell.BooleanCellValue);
            break;
        case CellType.Error:
            newCell.SetCellErrorValue(oldCell.ErrorCellValue);
            break;
        default:
            break;
    }
}