Hi,
Welcome to the site!
Most of this is already covered, but here is my method....if the sand in the 55g is older then 8 months, it needs to be rinsed. If the sand hasn't been sitting in the tank under rocks for any real time, there isn't any toxins to concern yourself with.
If you rinse your sand...do NOT use freshwater, use the saltwater from the 55g and finish a second rinse with some new saltwater. That way you won't kill off the good bacteria built on it. There is extremely little good bacteria in the water, so putting all new saltwater in the new tank with the established rock and sand...as well as FOR NOW, use the HOB filter along with the new wet-dry, and use all of old media, don't change a thing in it. You should have enough good bacteria, and have no cycle at all. The extra water volume will be just fine. ONLY use your established rock FOR NOW, unless the new is fully cured. If needed, cure the new rock in a separate container, or the old 55g, and add it later.
Acclimate your fish to the new digs as if they just came from the store. Anytime you change a critter from one tank to another, it's a safety precaution, because SW critters just do not handle change easy, everything will be very stressed. Their entire world has changed.
I have done 3 upgrades, and quite a few moves over the years, and the only thing I ever lost was a cleaner shrimp, which was odd since the other cleaner shrimp, his buddy, was fine.
