How do I update with a newly-created detached entity using NHibernate?

Question

Explanation:

Let's say I have an object graph that's nested several levels deep and each entity has a bi-directional relationship with each other.

A -> B -> C -> D -> E

Or in other words, A has a collection of B and B has a reference back to A, and B has a collection of C and C has a reference back to B, etc...

Now let's say I want to edit some data for an instance ofC. In Winforms, I would use something like this:

var instanceOfC;

using (var session = SessionFactory.OpenSession())
{
    // get the instance of C with Id = 3
    instanceOfC = session.Linq<C>().Where(x => x.Id == 3);
}

SendToUIAndLetUserUpdateData(instanceOfC);

using (var session = SessionFactory.OpenSession())
{
    // re-attach the detached entity and update it
    session.Update(instanceOfC);
}

In plain English, we grab a persistent instance out of the database, detach it, give it to the UI layer for editing, then re-attach it and save it back to the database.

Problem:

This works fine for Winform applications because we're using the same entity all throughout, the only difference being that it goes from persistent to detached to persistent again.

The problem is that now I'm using a web service and a browser, sending over JSON data. The entity gets serialized into a string, and de-serialized into a new entity. It's no longer a detached entity, but rather a transient one that just happens to have the same ID as the persistent one (and updated fields). If I use this entity to update, it will wipe out the relationship to B and D because they don't exist in this new transient entity.

Question:

My question is, how do I serialize detached entities over the web to a client, receive them back, and save them, while preserving any relationships that I didn't explicitly change? I know about ISession.SaveOrUpdateCopy and ISession.Merge() (they seem to do the same thing?), but this will still wipe out the relationships if I don't explicitly set them. I could copy the fields from the transient entity to the persistent entity one by one, but this doesn't work too well when it comes to relationships and I'd have to handle version comparisons manually.