This is what I did when I upgraded from a 75 to a 180, whole process took 2 days start to finish.
Picked up a 100G Rubbermaid stock tank, used this to cure new rock and holding tank during upgrade.
Did weekly water changes to swap tank water into the stock tank, tried to match water best I could over the 6 weeks to cure the new rock.
Filled 32 gallon BRUTE container with tank water.
Moved corals to a 20L (was and still is my QT tank) 50/50 fresh to tank water.
Moved all rock and fish from tank to tub, gave the rock a quick rinse in a 5G pail of tank water as well.
Broke the tank completely down and setup new tank.
Filled new tank with a little over ? way, matched temp and SG and moved all the rock in.
Added new sand and a few cups of the old sand bed to seed.
Added tank water from the BRUTE into the tank and started re-filling for a WC.
Let system run for the day to let the sand storm settle.
Moved fish and coral back in at night.
Topped the tank with fresh and tub water until normal run volume in the sump.
Just remember to keep lots of towels handy as you will spill lots of water, not to mention splash back from the fish as you move them.