Take the fish out
when i made the switch over to sand, i removed the fish and put them in a rubbermaid bin. I also ran the heater in the bin, as well as the filter and air stone. Also, put your plants in the bin as well, to keep them wet.
empty the entire tank, ALL the water. if you don't have the buckets to keep it, just get rid of it (water). save the top layer of gravel and put it in a nylon stocking and tie it up. then put that nylon stocking in the bin with the fish. then take the rest of the gravel out, clean teh tank if you want and put the sand in.
Saving the top layer of gravel will save the benefical bacteria. You will put this in the tank once it's back together to keep the bacteria colony alive until it settles in your sand and on the plants. You will only need to keep this in the tank a week or so.
When you are ready to put water in the tank, just use new, de-chlorinated water.... but FIRST, put a plate on the sand and pour the water on to that, slowly. This way, it will NOT stir the sand up, and you will not have cloudy water. I have done it this way a number of times, and it works great, even with the cheapest, dirtiest playsand.
then, put your heater in the tank, set up your filter and you are ready to go. I prefer to add the fish when the heat is somewhere close to where it should be. I have also solved this by using warm water when I re-fill my tank. Just don't add the fish to a cold, cold tank. Like they need any more stress at this point!
Good luck!

you can email me if you have any other questions....