I think there is a conceptual issue here. What you are trying is analogous to casting a Dog object to a SpottedDog object, which is bad--a Dog is not always a SpottedDog. I suggest you remove the existing Dog object, then save the SpottedDog object. For example: Dog d = em.find(Dog.class, ID); | SpottedDog sd = new SpottedDog(d); | sd.set*(...); | ... | em.remove(d); | em.persist(sd); //you could also use em.merge(sd) here View the original post : http://www.jboss.com/index.html?module=bb&op=viewtopic&p=4049360#... Reply to the post : http://www.jboss.com/index.html?module=bb&op=posting&mode=reply&a...