using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var legacyCustomers = new List<Customer>{ 

            new Customer() { Name = "Customer 1" },

            new Customer() { Name = "Customer 2" },

            new Customer() { Name = "Customer 3" },

            new Customer() { Name = "Customer 4" }

        };

​

        var currentCustomers = new List<Customer>{ 

            new Customer() { Name = "Customer 1", Source = "migrated" },

            new Customer() { Name = "Customer 2", Source = "migrated" },

            new Customer() { Name = "Customer 3", Source = "" }  

        };

​

        // get a list of customers that need to be migrated or updated to the new system (need to get Customer3 and Customer4)

        var migrateList = legacyCustomers.Where(

            c => currentCustomers.All(c2 =>

                !string.Equals(c2.Name, c.Name, StringComparison.CurrentCultureIgnoreCase) ||

                (string.Equals(c2.Name, c.Name, StringComparison.CurrentCultureIgnoreCase) &&

                 !string.Equals(c2.Source, "migrated", StringComparison.CurrentCultureIgnoreCase))

                // todo: add source conditional                   

            )

        ).ToList();

        foreach(var c in migrateList)

        {                

            Console.WriteLine(c.Name);

        }

    }   

}

​

public class Customer

{

    public string Name { get; set; }

    public string Source { get; set; }

}