Customer.cs

namespace ConsoleApplication1
{
    public class Customer
    {
        public int Zipcode { get; set; } 
    }
}

Program.cs

using System;
using Nest;

namespace ConsoleApplication1
{
    public class Program
    {
        public static void Main(string[] args)
        {
            var connectionSettings = new ConnectionSettings(new Uri("http://localhost:9200/"));
            connectionSettings.SetDefaultIndex("customers");
            var elasticClient = new ElasticClient(connectionSettings);

            elasticClient.CreateIndex("customers-v1");
            elasticClient.Alias(x => x.Add(a => a.Alias("customers").Index("customers-v1")));
            elasticClient.Map<Customer>(d =>
                d.Properties(p => p.Number(n => n.Name(name => name.Zipcode).Index(NonStringIndexOption.not_analyzed))));

            elasticClient.Index(new Customer { Zipcode = 8000 });

            var reindex =
                elasticClient.Reindex<Customer>(r =>
                    r.FromIndex("customers-v1")
                        .ToIndex("customers-v2")
                        .Query(q => q.MatchAll())
                        .Scroll("10s")
                        .CreateIndex(i =>
                            i.AddMapping<Customer>(m =>
                                m.Properties(p =>
                                    p.String(n => n.Name(name => name.Zipcode).Index(FieldIndexOption.not_analyzed))))));
            var o = new ReindexObserver<Customer>(onError: e => { });
            reindex.Subscribe(o);

            elasticClient.DeleteIndex(d => d.Index("customers-v1"));
            elasticClient.Alias(x => x.Add(a => a.Alias("customers").Index("customers-v2")));
        }
    }
}