Replace Shape3D in scene graph

[Dave]

I have a method that generates a Shape3D. I want to regenerate that shape
and replace the Shape3D originally added to the scene graph with the newly
generated one. How do I do this?

tg = new TransformGroup();
tg.addChild(generateShape());
rootBranchGroup.addChild(tg);

I want to replace the Shape3D added to tg with a new one created by
generateShape().