When I switched over from an undergravel filter to sand substrate and powerfilter/canister set-up I placed my fish in a 5g bucket along with the heater and the cannister filter. I then changed out the gravel for the sand and also painted the back of the aquarium black. I then filled, heated and treated the tank with water and transfered the fish and equipment back over. It took about 2 1/2 hrs. Most of that time was waiting for the paint to dry between coats.
I had no problems at all with the fish and really like the way things turned out.