I would probably put together an idea of structure then drop the new rock in sump for a couple/three weeks. Once cured or seeded Id then pull it, assemble it however you plan with cement or acrylic rods then move to display.
You could always take a hammee to the old structure once out of display and drop the pieces back into the sump to be pulled slowly if you’re worried about throwing things out of balance.
You could always take a hammee to the old structure once out of display and drop the pieces back into the sump to be pulled slowly if you’re worried about throwing things out of balance.