Going to planted is easy because you will be offsetting your bacterial biological filter with plants. So even though you lose some of your biological filtration you make up for it with live plants that will consume the ammonia.
I would not recommend filling the tank with tap water that has not been dechlorinated, as that will kill beneficial bacteria on the walls of the tank. I would prep your sand (I also recommend pool filter sand) the day before as it takes a LONG time to properly wash all of the finer particles that cloud the water. Work until you think it's clear, then work another 10 minutes, trust me....
Now put the sand aside until the next day and rest.
You want to have your plants purchased and ready to go on transfer day. Get a clean rubbermaid container and transfer over water from the tank (I would just do a gravel vac, but instead of doing what you normally do pull the water from the top since you don't want any detritus in the holding tank). Put your filter and heater from the tank into the rubbermaid container. Now you have a temporary tank that is just as good as a normal tank. Only after all of that has been done should you remove the decor from the tank and carefully transfer the fish (makes it much easier to do when all decor is out). Keep the container covered to prevent stress to the fish. Since you have a properly heated and filtered tank you can take your time and not have to worry about harming the fish/filter.
Get a bucket of tap water and add your dechlorinator. Remove the remaining water in the tank and gravel (be careful not to scratch the glass). There should be nothing left in the tank. Now add 1/2 the bucket of water, swirl around, and dump so any leftover particles and detritus can be removed. You should now have a perfectly clean tank but *WITH* bacteria on the surfaces that you want to keep alive.
You can do one more quick wash of the sand if you want, otherwise carefully dump it in (it's easier if you add it in when it's almost dry). Now add dechlor for the total tank volume and refill the tank to ~75% (you want some room to work while adding in plants/decor). Scape the sand how you want (a lot of people like the back to be higher than the front, but I find within a week or two the sand evens out anyways so I just do mine level).
Depending on how much you trust your new live plants (I don't) you can either add them in directly or first perform a tap water soak or bleach/hydrogen peroxide dip. This will kill the beneficial bacteria on the plants but also any potential parasites/algae along for the ride. Now do your planting.
When you're all done and happy with the look depending on how cloudy the water is you may want to do a large water change.
When everything is finished transfer the filter and heater from the container back into the tank and make sure everything is working. When you're sure it's OK now move the fish back into their new home. I'd keep the tank lights off for the day and not feed to reduce stress on the fish and filter.
HTH
