using System;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        var list = new List<string>{"a", "b", "c", "d", "a", "b", "c", "d"};

        var result = list

            .ReplaceAll("a", "XXXX")

            .ReplaceAllIf(true, "c", "YYYY")

            .ReplaceAllIf(false, "d", "ZZZZ");

        foreach (var x in result)

            Console.WriteLine(x);

    }

}

​

public static class Extensions

{

    public static List<T> ReplaceAllIf<T>(this List<T> list, bool condition, T valueToFind, T replacement)

    {

        return condition ? list.ReplaceAll(valueToFind, replacement) : list;

    }

​

    public static List<T> ReplaceAll<T>(this List<T> list, T valueToFind, T replacement)

    {

        return list.Select(x => x.Equals(valueToFind) ? replacement : x).ToList();

    }

}