You can use all the old water, rock and sand. But when removing the water try not to disturb the sand bed to much. Doing so will not only make a huge mess, but could also release nitrates which can cause a cycle. You might not be bale to re use 100% of the water as when moving water/rock you will disturb the sand bed no matter what. I would move as much as possible and swish the rock intot he water your not using to rinse all the detritus off of it.  After the transition I would keep a close eye on the levels by testing daily.