What I did in my 55 might help.
I started by putting everything to one side so I had space to work. I siphoned up the gunk under the barren gravel bed with a gravel vac. Then I took that gravel out and threw it in a bucket. I used a piece of plastic mesh to hold back the crushed coral. I dosed my tank with bacteria and filled the empty area with sand. In a few days, I moved some rock to the sand bed and did another small area, moving the plastic mesh. I did my whole tank in 4 or 5 batches, each a few days apart. I got a small NO3 spike, but it went back down in 2 or 3 weeks.
I did this in batches because I thought it was important to preserve the bacteria in the substrate *and* to let the new sand populate for a few days before putting anything on it. At the time, I had about 90 lbs of rock in my 55. I didn't have much room to work in, but I was able to do this without any problems. I have softies and LPS only. If I had SPS, I would take them out to avoid NO3 problems.
I think if you take everything out at once, you're going to stir up the gunk and have no bacteria to digest it. The tank would have to cycle again (half cycle?) and that could be bad.
Hope this helps!
