It's normal to miss significant conceptual classes during early domain modeling, and to discover them later during design sketching or programming. If you are taking an agile modeling approach, the purpose of creating a domain model is to quickly understand and communicate a rough approximation of the key concepts. Perfection is not the goal, and agile models are usually discarded shortly after creation (although if you've used a whiteboard, I recommend taking a digital snapshot). From this viewpoint, there is no motivation to maintain or update the model. But that doesn't mean it's wrong to update the model.
If someone wants the model maintained and updated with new discoveries, that's a good reason to redraw the whiteboard sketch within a UML CASE tool, or to originally do the drawing with a tool and a computer projector (for others to see the diagram easily). But, ask yourself: Who is going to use the updated model, and why? If there isn't a practical reason, don't bother. Often, the evolving
domain layer of the software hints at most of the noteworthy terms, and a long-life OO analysis domain model doesn't add value.