Is it possible to move an entity from one collection to another?

I have two top level entities A and B which both have one-to-many
references to another entity C, which happens to have its own network
of stuff below it.

Cascade deletes are working fine - if I delete C, it and everything
below it goes away.  Also, if I delete A or B, then it and its
children also go away.  So far so good, and I want to keep this
cascading behavior.

Now the interesting part.

I want to take C, remove it from A's one-to-many collection and add it
to B's one-to-many collection, preserving the object network under c.

 a.getChildren().remove( c );
 c.setParent(null);
 b.getChildren().add(c);
 c.setParent( b );

I get errors like "Found two representations of same collection" the
moment I do the add().

I'd think this would be a trivial reassignment of some foreign keys,
but there seems to be more to the story.  Anyone know what it is?
Does Hibernate assume remove() is a delete?  The object c is not being
orphaned.

Thanks,
-Walt Stoneburner, wls@wwco.com