I've done the switch over from gravel to sand when I had my 10g... its a long process so you better have a full day to dedicate to it. Thats why I started out with playsand when I upgraded to my 33g long. The trick is to have a temporary holding tank that you can have your fish in while you do the switch.
I had an old 5g bowfront from my first goldfish years back, so I cleaned that out and used it to hold the fish while I did the switch. Since it was half the size of my 10g, it was easy to syphon half the water out of the 10g directly into it, switch my filter over to it, net up all the fish and transfer them over... and then I had a tank with nothing but gravel and water to work with.
The key is to NOT gravel vac when you are syphoning water into your holding tank, but when you get the remaining water out get as much of that gunk out as you can, it makes cleaning the gravel out a cleaner process. Once you get all the water it, just scoop up the gravel, and hose out the tank and clean it up as much as you'd like.
Next is the frustrating part. Fill your tank about 1/4 full with water, then put in your sand (which you have hopefully pre-washed beforehand). Now to fill the rest of the tank get a bowl which is easy to keep a hold of, because you need to keep the lip of the bowl at the waters surface and as you fill the rest of your tank pour the water into the bowl and let it overflow out. This will keep you from mixing up as much sand as possible from the bottom.
Now that you have a full, cloudy tank its time for water changes! Keep taking out water and replacing it a bucket at a time until it starts looking clear! This process took me a few hours to do all by itself both times I've had to do it. Its frustrating, and quite the workout if you are lugging buckets and don't have the luxury of spending $70 on a python just to do water changes. In the end though, this is the quickest way to clear up the water. Do not be tempted to say "screw that" and just run your filter to clear it up... it will take forever, OR kill your filter...
Well I hope that helped, if I left out any details that may confuse you don't be afraid to ask.