class ContainerV1
        {
            public string Id { get; set; }
            public bool A { get; set; }
        }

        class ContainerV2
        {
            public string Id { get; set; }
            public bool A { get; set; }
            public SubItem[] Blah { get; set; }
        }

            var mongo = new MongoClient("mongodb://localhost");

            // start from scratch
            mongo.DropDatabaseAsync("foo").GetAwaiter().GetResult();
            var test = mongo.GetDatabase("foo");

            // insert ContainerV1 record (without array member)
            var coll1 = test.GetCollection<ContainerV1>("bar");
            var item = new ContainerV1 { Id = Guid.NewGuid().ToString("N"), A = true };
            coll1.InsertOneAsync(item).GetAwaiter().GetResult();

            // get from same collection after model changes.
            var coll2 = test.GetCollection<ContainerV2>("bar");
            var results = coll2.AsQueryable().Where(i => i.A).Select(i => new { Blah = i.Blah }).ToArray();

{ "aggregate" : "bar", "pipeline" : [{ "$match" : { "A" : true } }, { "$project" : { "Blah" : "$Blah", "_id" : 0 } }], "cursor" : { } }

{ "cursor" : { "id" : NumberLong(0), "ns" : "foo.bar", "firstBatch" : [{ }] }, "ok" : 1.0 }