The tank will probably experience changes with any replacement of sand or addition of rock. Since you already have live rock, adding dry rock would better since it does not contain the die off that can cause a mini cycle. However dry rock will take time to become live and you should be keeping a close eye on ammonia, nitrite and nitrate during this process.
Changing out you sand should be no problem. Again using dry sand will avoid ammonia spikes. Live sand will have dieoff.
29g Mixed Reef, 150w 14k Phoenix MH w/ Moonlights, Reef Octopus BH1000 HOB Skimmer, Ecotech Vortech MP10es, ReefKeeper Lite (Plus), PH Probe, Temp Probe, ATO, Carbon/GFO Reactor.