Actually, I think this is not what the asker meant. I just came across the same question, whether i can e.g. select some nodes of my graph (which may be of type rounded rectangle shape node) and then "transform" them into another shape, e.g. ellipse. In my perfect world, I would also like to be able to transform between different subpalettes, i.e. transform a shape node into a group node, as an example.
PS: I guess one would call this a "replace"-function.