I left my fish in too when i changed over,more or less the same as proxo,except i got a nice thick tube and sucked the gravel out,it also picked up the debris up underneath the gravel,then when putting sand back in,make sure you've washed it until its clean,then give it another go once more just to be sure,the cleaner the sand the less cloudy the tank will be.
I used a large jug to put the sand in,let the jug fill slowly with tank water until its full with water and sand,then gently lower it down to the bottom and pour,if you do it gently it won't create a snow storm.
Remember to keep the filter off during this time.
Other peeps have removed the fish into a large plastic tubs with the heater and filter in there with them,but obviously has i've never done it this way i dont know how long the process takes.