[BsonKnownTypes(typeof(B))]
public abstract class A
{

}

public class B : A
{

}

public class C
{
    [BsonElement("a")]
    public A A { get; set; }
}

class Program
{
    static void Main(string[] args)
    {
        var b = new B();

        var s = Update.SetWrapped("a", b);
        var t = Update<C>.Set(c => c.A, b); // throws BsonSerializationException
    }
}