A friend just called and asked about this...here you go man...
*Have new tank ready to transfer all from existing tank. Ie; RO Vinegar wipe down inside tank, Ro rinsed carbon or whatever media you will use in your new filter.
***If using the same equipment such as power heads, heater, filter...it would be good to clean them in RO Vinegar water to get rid of junk... algae, Aiptasia, etc. Then just RO rinse and they are good. Place the equipment in new tank where you want it to be.
*Have new RO Salt water ready and temp at the same degree range as the existing tank... Simple to do by having it in covered container(s) with a heater in it. ***Make sure the salinity and pH are the same as your existing water in 'old tank'. (always try to have a little more of both RODI Salt water as well as RODI fresh if adjustments need to be made for volume and/or SG tweaking.
*Have several containers; 5 gallon buckets, Rubber Made trash cans or some kind of clean tubs on hand. Amount of containers will vary according to how much volume you are going to store.
*Remove LR from existing tank and place in bucket(s), tub(s) and dip existing water into container with a pitcher, or use pump with flex hose to fill. Again, depends on what size you are doing as to the fastest way possible.
*Remove water to fill bucket about with about X amount of gallons of established water and place critters in containers covering with egg crate so no jumpers or any critter gets out. If you're not going to an extremely large tank I don't believe it will not be necessary to keep a heater in the container(s) as it will not take all that long to do it. You can keep an air stone in the bucket with critters or just stir the water if this 'switch' runs longer than an hour or so. If big project, have heater if necessary and circulation pump.
*Remove remainder of water to bucket/container leaving about X amount gallons of established water in tank with existing sand.
*Stir, stir and stir sand which will turn almost black from debris, nitrates, phosphates and all the bad junk which would create a crash if you just moved the sand 'as is' like some do to another tank without bringing the bad stuff to the water instead.
*Start dipping the bad water from old tank to a 'waste' container as you will be tossing this water. You will lose a few pods but your critters will be fine. While doing this stir some more, push the sand to the side of tank allowing the water to drain from sand and have more area to dip the remainder of bad water out...continue until you have the bad water out.
*Dip sand out with something like a large plastic drinking glass and pour off whatever water is at the top of the cup. Have a container to hold your LS. It will be fine for the little time it's going to take you to switch tanks...if it's going to be an all day project it is wise to have water atop the sand with mild circulation.
*Hard part is done and now the fun and rewarding part...This will go quick.
*In the tank you will be putting them in, add around 50% or so of NEW RO Salt water, does not need to be exact, but close. Put in LR, add sand with the cup you used to remove the sand from old tank and slowly submerge ti to new rank pouring it around in the tank around the LR. If you are adding more LS, it would be fine to put in at any point either with your existing sand, before or after it does not matter. It would also be wise to add double dose of 'Prime' to ZERO out any bad juju remaining.
*Add the remainder of established water you have contained to new tank slowly. Should have very little cloudiness and 'sandstorm'. At this point I admit, I use some Stability by Seachem. In your new tank would add caps of it according to directions...it will mix in with no issues. Probably not necessary but this is what I have always done with success.
*Add NEW RO Salt water to tank to capacity. ***I do not have power on yet as to not stir anything around as it's already got plenty of oxygen in it from adding the water.
*Remove critters however you normally do and place in new tank. If you have an anemone it would be good to place it where you like. ***Do not use the gallons of water from this container as it will be full of NH 3 at this point...simply dispose of it.
*Turn power on and make sure power heads, filter, etc are working.
*******What you have done is essentially is a good cleaning as far as all is concerned by using established everything removing the bad gunk which eventually leads to old tank syndrome and possible crashes in old tank at some point and ~50% water change. Your critters will be happy in a clean, bigger, or different home with no ill side effects.
Enjoy the new, yet established home.