I suggestion would be this. Put your live rock and animals in a bucket. Put your water in buckets. When you are down to a couple of inches of water, stir your substrate up a little to get the gunk out of it. then syphon the water out with the gunk, get rid of the water from the stir. Then put your sand into the new tank and put most of the water(except what your animals are in) in too. Put in your live rock then add new fresh saltwater to almost fill it up. Then turn on the filters for a little while to let the water clear before adding the fish and inverts. Either use the existing filter or move the existing filter media over to the new tank. Be ready with enough water for a water change and keep a close eye on the ammonia. You may have a slight amm spike but not much if at all. You will have the bacteria from the rock, sand, and filter. should be enough IMO
. Be sure you don't rinse the sand in fw
, just stir it up in the last of the existing water.