Well, your tank's stocking concerns aside, switching isn't too hard. Get the sand that you want first and clean it. Do that by pouring it into a 5 gallon bucket about 1/2 bucket at a time. Then shove a garden hose in and turn it on full. Keep jamming the hose down into the sand and move it all around. Anything that swirls over the bucket's rim you didn't want anyway as it was too light. Once the water's clear no matter what you do to the sand, that bucket's done and you can begin on the next until you have all your sand washed. Total for a 55 gallon, in my opinion, is about 40-45 lbs. of sand.
Once all the sand is clean, make sure you get it inside and warmed up to a close approximation of your tank's temp. This will prevent your water temp from plummeting when you put it in the tank. Now, you have a decision to make: whether or not to take all the fish out of the tank before beginning. If you do, then simply put them somewhere they can survive at least a few hours with a heater and an air pump or something (I highly suggest doing this as it's probably less stressful on them and easier on you). If not, you'll be doing half the tank at a time.
The next process is when things get messy. Make sure all filtration is off during this process and drain about 1/2 to 2/3rds of the water. If all the fish are out already, simply scoop all the gravel out of the tank, or siphon it out through a largish hose. If they aren't, I'd suggest putting one hiding spot down on the side of the tank you'll not be working on yet so the fish will gravitate to that side and more or less stay out of the way. Once you have one side done, simply move the decoration to the bare-bottomed side of the tank and begin on the other side. Eventually you'll have all the gravel out and the fish'll either be stressed out in their now-empty tank or in their temporary tanks not knowing what's going on but still stressed out. Now you want to add the sand into your tank. Do this by scooping it down into the water or simply lowering the 5 gallon bucket down into the water on its side and scooping the sand out sideways into the tank...not by just chucking it in or pouring it from on high. This will keep the majority of it from getting stirred up into the water column. When the sand is there, spread it around how you'd like, add decorations back in and refill the tank making sure the temp is where you want it at all times. Wait for the sand to settle a decent amount before turning on your filters and make sure the intakes are at least 1/2 way up the tank's sides, not down near the bottom.
As far as cleaning you simply suck stuff off the surface of the sand. There's a lot of good videos of how to do that out on youtube if you want. I stir my sand about every 4 weeks or so. If you want Malaysian Trumpet Snails in your tank, they do a good job stirring it for you. Good luck and enjoy!
