gsp if you're only keeping abit of sand then this should be easy just empty contents of the old tank to the new, leave sand and abit of water in the old then take out what sand you want and remaining water with 3 or 5 salt pails, rinse the sand going from one pail to the other and so forth you know what I mean!
in the end all the detrtious should be minimal then cut off all the flow in your new tank, bag the sand in plastic bags drop it in the tank and gingerly pour them out causing minimal disturbance, alternatively poke a hole in the bottom of the bag and use the bag like a pastry chef piping a cake with icing TA DAH!!
