I think Revit could allow to change the category of system families. For Example, we can model a ceiling using it's features, and after this, we can change the instance to a floor instance. Or, we can model a roof , using slope and etecetera, and after we decide that this element just will not be a roof anymore, but a floor, and this is already modeled, why can we just change the category? I think it will be awsome!
