Sorry for the long post
I changed my substrate a couple of months ago, and all i can say is i am glad i did. My plants are now thriving, my water is clearer than it has ever been and its much easier to keep, with minimal algae growth. I changed from sand to a multi layer substrate of peat pads, vermiculite, laterite, sera floredepot and gravel. I had been planning it for a while, so when the day came i knew exactly what i had to do, and everything went smoothly.
To prepare, I bought all the new products to make up my substrate and washed them. I got my new driftwood and boiled it and also some fish bags. I also bought a kitchen style plastic bin of about 60 litre capacity, I have a 110 litre tank, so I could keep half my water. If you need to perform any maintainence on your tank or stand, now is the time to do it, while the tank is empty. I took the opportunity to brace the back of my stand and paint the back of the tank black with some quick drying matt black spray paint.
On the day, I got up early (the entire process took about 12 hours). I started by removing half of the water from the tank to the bin. When I did this I placed a piece of filter floss over the tank end of the hose to filter out any particles. I then placed the heater and filter media in the bin. Keep the heater on. Its wise to keep the bin close to the tank for power and ease of transferring water and fish. I also removed any plants and decor from the tank, cleaning and placing any i wanted to keep in the bin. It also provides hiding places in the bin for any fish that are stressed.
Once all the decor and plants were out, I caught the fish and placed them in the bin. Its easier on you and the fish with less water and no decor. Now the fish are in, cover the bin with a, or a towel. This will keep the heat in, the light out to help them feel less stressed, and stop them from escaping.
Now back to the tank. Empty the rest of the water out and chuck it. Clean out your old substrate and clean the tank. Now is the time to perform any maintainence or modifications to your setup. Once you have the tank set back up again, you can put in your new substrate that you cleaned before the whole process. You can also place your decor where you want it. Fill the tank half way with water which has been raised to the correct temperature and dechlorinated. You want to do this without stirring up the substrate. I planted some plants at this point too.
When you put the fish back, you will have to acclimate them. In the bin, catch your fish and put them in fish bags. Trasfer the bags to the tank and leave them for the moment. I then transferred the remaining water from the bin, along with the heater. I got the filter running in the tank again too. Once all the water is back, leave it for a while to let the water mix and the temperature settle. You can then acclimate the fish as you would normally.
This worked very well for me and went smoothly, because I prepared everything I would need and planned ahead. My tank is now looking and working better than ever. Good luck!