So, really if the Great Houses can force regeneration upon themselves and if they can affect the way in which they change, what's to stop them from becoming, say, a bunch of trees? They could all turn into living rocks. Or ponds. They could become beings of pure energy.
Actually, what's to stop someone else from sculpting the regeneration to their will? Or triggering it for that matter? Could they make them become whatever they want? Could the Enemy walk up and effectively turn a Homeworld operative into a frog? Are all accounts of such witchcraft actually just this happening?