I have a Merchant object. It is subclassed by two objects: ProspectMerchant and ClientMerchant via a joined-subclass. Other objects, like documents and notes are attached to the Merchant. Throughout the life cycle of the system, a ProspectMerchant will be come a ClientMerchant.

What is the correct way to make the transition between one subclass to another? I need to preserve those links to the other objects.

This may be completely obvious, but I'm a newbie when it comes to NHibernate.

To answer your question you have to first realize that converting form one sublcass to another is impoosible in C# (or Java and many other languages for that matter).

The truth is you should probably look very closely at your design because I think you are confusing certain OO concepts. The kind of "issue" you have with a class mutating are usually solved with a Role pattern.

Good luck and feel free to ask again when you delved a bit more into your design.

You could put a static method on ClientMerchant, something like FromProspect(ProspectMerchant), and return a new ClientMerchant object after populating all the properties from the ProspectMerchant.

Or, you could drop the whole heirarchy stuff and put a boolean property IsProspect on your Merchant object. It would start as true and get flipped to false at some point in the system's lifecycle. This assumes, of course, that there are no other differences between all three objects here.