When I changed my 55 over to sand from gravel, I still had fish in it.
I cleaned the sand really well using a 5 gallon bucket (I had to do this twice for the volume of sand I wanted). I poured the sand about 1/2 way up, then put a garden hose down into it and stirred it around. The water flushed the sand out until it ran completely clear. Once it was clean, I poured the sand into a trash bag and did the other 1/2. 50# total.
Next I brought the sand in to warm up for a day after the cold water rinse. The next day I lowered the water to about half or so, turned off the filters, then scooped out all the gravel I could with my hands. After that, I used a soft brush and a dust pan for the rest. Once I had all the gravel out, I used a siphon to do another quick clean up of miscellaneous gunk that was in the gravel.
Taking the entire garbage bag of sand, I lowered it into the remaining 1/3rd of the water on one end then gently slid the bag out from around the sand leaving a pile of it. Then I gently pushed the sand where I wanted it without stirring it up a lot. After placing my decorations back in, I refilled the tank. I aimed the water stream onto my piece of driftwood so it wouldn't stir up the sand. I then put my filter media into the water to float for until the little bit of sand that had been stirred up could settle. About 4 hours later, I turned the filters back on.
That's all there was to it.