Hi, I would convert to the wet/dry first and let it get established with all the good amonia and nitrite reducing bacteria for about six weeks before you change the bottom out to live sand. You don't want to do the other first as the established bacteria colony in your wet/dry will help to prevent a harsh cycle in the tank. It may still go thru a mini-cycle when you change out the CC. When you clean your wet/dry, only clean part of the biomaterial at a time and only wash them with salt water....not chlorine water. You don't want to kill the good bacteria. I have heard that some people divide bioballs up in sections with net material and only clean one section at a time. Also get yourself a bottle of cycle bacteria to add extra bacteria to help prevent the tank from cycling as much. The live sand will also help. Hopefully you have a lot of cured live rock.....this will help too. Watch your amonia and nitrates often during the first month or two and have water on hand to do some partial water changes if the levels start to go bad on you. You may need to do more frequent water changes for a while, but if you are not over stocked you fish should be ok. I would place my fish in those five gallon buckets with an airstone and the lid partially covering to keep the fish from jumping out while you switch out the CC. Don't try to cram them all into one bucket! Make sure you keep the live rocks wet too.
Good luck, Lesley