replacing characters in strings

Try this:

using System;

class Program
{
    static Random rand = new Random();

    static void Main()
    {
       char[] vowels = new char[]{'a', 'e', 'i', 'o', 'u'};
       char[] consonants = new char[]{'c', 'g', 'j', 'k', 'q', 's', 'x', 'z'};
       char[] uVowels = new char[]{'A', 'E', 'I', 'O', 'U'};
       char[] uConsonants = new char[]{'C', 'G', 'J', 'K', 'Q', 'S', 'X', 'Z'};

       string text = "The Quick Brown Fox Jumps Over The Lazy Dog";
       Console.WriteLine(text);

       char[] temp = text.ToCharArray();
       int index = -1;

       for(int i = 0; i < temp.Length; i++)
       {
          if ((index = Array.IndexOf(vowels, temp[i])) > - 1)
          {           
             temp[i] = GetDifferentRandomChar(index, vowels);
          }
          else if ((index = Array.IndexOf(consonants, temp[i])) > -1)
          {
             temp[i] = GetDifferentRandomChar(index, consonants);
          }
          else if ((index = Array.IndexOf(uVowels, temp[i])) > - 1)
          {           
             temp[i] = GetDifferentRandomChar(index, uVowels);
          }
          else if ((index = Array.IndexOf(uConsonants, temp[i])) > -1)
          {
             temp[i] = GetDifferentRandomChar(index, uConsonants);
          }

       }

       string text2 = new string(temp);
       Console.WriteLine(text2);
       Console.ReadLine();      
    }

    static char GetDifferentRandomChar(int index, char[] chars)
    {
       int len = chars.Length;
       int next;
       do
       {
          next = rand.Next(0, len);  // excludes 'len'
       }
       while (next == index);
       return chars[next];
    }
}