Theory= If the current rock is supporting your bioload, moving everything (minus detritus cleaning) should not be a problem. The dilution factor of +100 gallons of water should improve the room animals have to roam and lower the immediate impacts of their normal biolgical activities upon water qualities.
I would get a Brute or old an old tank and put the dead rock in with a small circulation pump in new water* and measure initial concentrations of Alk, Phos, nitrates. After it has soaked for a few days test again to see if the dead has impacts upon the clean water parameters. If you use dry quarry rock I would expect to see a drop in Alk and a little Phos & possibly nitrates. If the Phos is significant I would add lanthanum chloride to the barrel and let it soak awhile to remove that. If it drops alk, I would add an excess of baking soda to the mix to get those empty spots in the rock structure filled with alk.
I would recommend testing for copper if this is old tank rock that may have been used in a copper treatment.
Ultimately I would want to be assured of what was going into the tank that might impact water quality going forward.
IF you change your mind and go with live rock then I would do the same thing but look for live hitchhikers, crabs, sponges, corals, vermitids (if it's from another reefer), etc. Live corals I might add a light over the barrel, hth.
Edit. *Good place to start with brute water is from a water change, using your DT water (known parameters) for soaking the new rock and measuring what it is brining.
