using MongoDB.Bson;
using MongoDB.Bson.Serialization;
using MongoDB.Bson.Serialization.Attributes;
using MongoDB.Driver;

var client = new MongoClient("mongodb://***");
var db = client.GetDatabase("test");
BsonClassMap.RegisterClassMap<BaseClass>();
BsonClassMap.RegisterClassMap<DerivedClass>();
var collBase = db.GetCollection<BaseClass>("OfTypeUpdatePipeline");
var collDerived = collBase.OfType<DerivedClass>();

// Works
collDerived.UpdateOne(
    x => x.Key == "TestKey",
    Builders<DerivedClass>.Update.Set(x => x.Value, "Plain update"),
    new UpdateOptions() { IsUpsert = true });

var pipeline = new EmptyPipelineDefinition<DerivedClass>()
    .AppendStage<DerivedClass, DerivedClass, DerivedClass>(new BsonDocument
    {
        { 
            "$set", 
            new BsonDocument
            {
                { nameof(DerivedClass.Value), "Pipeline" }
            }
        }
    });
var update = Builders<DerivedClass>.Update.Pipeline(pipeline);
// Fails with: 'Unable to cast object of type 'MongoDB.Bson.BsonArray' to type 'MongoDB.Bson.BsonDocument'.'
collDerived.UpdateOne(
    x => x.Key == "TestKey",
    pipeline,
    new UpdateOptions() { IsUpsert = true });

public class BaseClass
{
    [BsonRepresentation(MongoDB.Bson.BsonType.ObjectId)]
    public string Id { get; set; }

    public string Key { get; set; }
}

public class DerivedClass : BaseClass
{
    public string Value { get; set; }
}