I would purchase the dry rock about a month before the upgrade and put it in a tub or Brute can with freshly made saltwater, heater, and powerhead so it can become cured and get all the phosphates out of it, changing the water every so often. Once ammonia reads zero you will be good to go. How old is the existing sand? If it over an year old, IMO I would just get all new sand. Now you might have a mini-cycle, but I don't think it will be nothing major.