Convert XLS to XLSX - C#
Spreadsheet SDK sample in C# demonstrating ‘Convert XLS to XLSX’
Program.cs
using System;
using System.Collections.Generic;
using System.Text;
using System.IO;
using Bytescout.Spreadsheet;
using System.Diagnostics;
namespace CSharp
{
    class Program
    {
        static void Main(string[] args)
        {
			// Open existing XLS file
            Spreadsheet spreadsheet = new Spreadsheet();
			spreadsheet.LoadFromFile("Input.xls");
            // delete output file if exists already
            if (File.Exists("Output.xlsx")){
                File.Delete("Output.xlsx");
            }
            // Save document
            spreadsheet.SaveAs("Output.xlsx");
            // Close Spreadsheet
            spreadsheet.Close();
            // open generated XLSX document in default program
            Process.Start("Output.xlsx");
        }
    }
}