I made the conversion on all but one tank. It's easier from scratch (on an initial setup), but, you can do the conversion as long as you have the patience to wait for the water to clear.
I like about an inch on the smaller tanks. About three inches in the back, tapered to about an inch and a half in the front on the 40g and above. The 5' 120g has about four inches in the back and tapers down to about 1 1/2" in the front.
You will be surprised how much more poop you will see with the sand. It's a little trickier to clean it up, but after a few water changes, you will get the hang of it with your python.
Bottom line...sand is better for the fish than gravel. Gravel is better than sand for the fishkeeper. :lol: