I kept the fish in when I switched. They were all ok. Just slowly scoop a net on the bottom to dip out the gravel. Only do 1/3 every other day so you don`t mess up the beneficial bacteria and start a mini cycle. The trick is to do it slowly and nothing gets stirred up.
You`ll love the sand, just skim the vac over top and remove all the waste. Oh and if you have big poopers like cichlids the poo will really show up on the sand so daily poo vacuuming is needed.