Error message when you try to change the name of a site column back to its original name in Windows SharePoint Services 3.0: "The column name that you entered is already in use or reserved."

using System;
using System.Globalization;
using Microsoft.SharePoint;

public class RenameField
{
    public static void Main(string[] args)
    {
        int i = 0;
        string strWebUrl = string.Empty;
        string strFldInternal = string.Empty;
        string strFldDisplay = string.Empty;
        while (i < args.Length - 1)
        {
            switch(args[i].ToLower(CultureInfo.InvariantCulture))
            {
                case "-url":
                case "/url":
                    strWebUrl = args[++i];
                    break;
                case "-fld":
                case "/fld":
                    strFldInternal = args[++i];
                    break;
                case "-display":
                case "/display":
                    strFldDisplay = args[++i];
                    break;
                default:
                    break;
            }
            i++;
        }
        if (string.IsNullOrEmpty(strWebUrl) ||
            string.IsNullOrEmpty(strFldInternal) ||
            string.IsNullOrEmpty(strFldDisplay))
        {
            PrintHelp();
            return;
        }
        SPSite site = new SPSite(strWebUrl);
        SPWeb web = site.OpenWeb();
        SPField fld = web.Fields.GetFieldByInternalName(strFldInternal);
        fld.Title = strFldDisplay;
        fld.PushChangesToLists = true;
        fld.Update();        
    }

    private static void PrintHelp()
    {
        Console.WriteLine("Reset the display of a field template " + 

                    "on the specific SPWeb:\n\n" +

                    "RenameField -url <absoluteWebUrl> " +

                    "-fld <FieldInternaName> " +

                    "-display <NewDisplayName>");

            
    }
}