C# Reset Index Customer ID (Windows Form Application)

using System;
using System.Collections;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Data.SqlClient;

namespace SampleSmartCardSDK
{
    public partial class frmReIndexCustomerID : Form
    {
        private SqlConnection objConn;
        private SqlCommand objCmd;
        private SqlDataAdapter _dtAdapter;
        private DataSet _dtSet;
        private String strConnString, strSQL;

        public frmReIndexCustomerID()
        {
            InitializeComponent();
        }

        private void ReIndexOfCustomerID()
        {
            strConnString = "Server = localhost; Uid = sa; PASSWORD = password; database = Customer;" +
                "Max Pool Size = 400; Connect Timeout = 600;";
            objConn = new SqlConnection(strConnString);
            objConn.Open();

            strSQL = "SELECT CustomerID FROM CustomerInfo ORDER BY CustomerID";

            _dtAdapter = new SqlDataAdapter(strSQL, objConn);
            _dtSet = new DataSet();
            _dtAdapter.Fill(_dtSet, "CustomerInfo");

            List<string> customerID = new List<string>();
            foreach (DataRow dtRow in _dtSet.Tables["CustomerInfo"].Rows)
            {
                customerID.Add(dtRow["CustomerID"].ToString());
            }

            for (int i = 0; i < customerID.Count; i++)
            {
                string prefix = "CUS";
                string posfix = prefix + (i + 1).ToString("0######");

                objConn = new SqlConnection(strConnString);
                objCmd = new SqlCommand("UPDATE CustomerInfo SET CustomerID = @customerIDNew" +
                    " WHERE CustomerID = '" + customerID[i] + "'", objConn);
                objCmd.Parameters.AddWithValue("@customerIDNew", posfix);
                try
                {
                    objConn.Open();
                    objCmd.ExecuteNonQuery();
                }
                catch (Exception ex)
                {
                    MessageBox.Show(ex.Message);
                }  
            }
            MessageBox.Show("ทำรายการเสร็จเรียบร้อยแล้ว", "จัดเรียงรหัสลูกค้า");
            Application.Exit();
        }

        private void frmReIndexCustomerID_Load(object sender, EventArgs e)
        {
            ReIndexOfCustomerID();
        }
    }
}