I was lucky when I done this, I had 2 80ltr storage units that i kept clothes in. I split the old tank water between these then dumped what would not fit. I then put a heater in one with a filter and in the other I put the tank plants and ornaments. Then came the best part, catching the fish lol. The sand had been in a bin with a hose pipe cleaning the water for the best part of a day so it was nice and clean. once the tank was cleaned up and ready I put the new sand in with the plants and other bits then started to siphon the water out of the container back into the tank, nice and slow so not to disterb the sand to much, next in went the fish and the remainder of water followed by topping up with new water. The tank was cloudy for about an hour but my 2 ex filters managed reakky well in clearing it with a little help from some filter floss. That was about 4 months ago now and I have never seen my fish so happy. I have 6 shrimp and they where fine with the moving about as where all the other fish.