I changed to sand this week. It's not something I'd like to do again in a hurry but if you want to do it here's a run down of what I did
1. wash the new sand really well (I used 30 kilo's for a 110 litre) and stored it in a large storage box. It took several hours to wash it all so forward planning is a good idea.
2. Moved my snails and oto's in to my spare tank and took out the decor - bog wood, plants, rocks.
3. started syphoning water, filled a 25 litre storage box with tank water and put the bog wood inside to keep it wet and to reduce fish stress. Whilst it was syphoning catch all the fish and added them to the storage box with the bog wood. Added a bubble stone and a heater and closed the lid (cover it with blanket/towels if it's a clear box to also reduce stress)
4. Started scooping out the old gravel with my fish net and chucking it in an old bucket.
5. once all the gravel had gone I chucked in some warm water and cleaned the glass, the bottom and around the filter box and syphoned it back out.
6. Get the hose out and started trickling in some water and adding the sand (I found it settled better by adding water slowly with it)
7. level off the sand to how I wanted it and added the plants back. Checked the temp and added a double dose of stresscoat and King British Safe Water (bottled bacteria - I've had good results with it in the past).
8. put the bog wood back and started re-adding the fish
9. switched off the lights for the rest of the day to give the fish chance to settle in.
10. Fed very lightly after a couple of hour but they wern't that bothered to be honest. They were investigating their new enviroment and a little stressed.
It took about 24 hours for the water to clear. I washed the life out of the sand but it still stayed cloudy.
If there's anythign else you want to know just ask