Like String.Replace but can do so with regards to case and culture.

ReplaceStringCaseInsensitive.cs

using System;
using System.Text;

public static class ExtensionMethods
{
    /// <summary>
    /// Case insensitive string replacement
    /// </summary>
    /// <param name="str">string to process</param>
    /// <param name="oldValue">old value to find</param>
    /// <param name="newValue">new value to replace</param>
    /// <param name="comparison">Case sensitive / insensitive</param>
    /// <returns>A string with the value replaced - case insensitively</returns>
    public static string ReplaceString(this string str, string oldValue, string newValue, StringComparison comparison)
    {
        var sb = new StringBuilder();

        int previousIndex = 0;
        int index = str.IndexOf(oldValue, comparison);
        while (index != -1)
        {
            sb.Append(str.Substring(previousIndex, index - previousIndex));
            sb.Append(newValue);
            index += oldValue.Length;

            previousIndex = index;
            index = str.IndexOf(oldValue, index, comparison);
        }
        sb.Append(str.Substring(previousIndex));

        return sb.ToString();
    }
}