I agree with Akasha. Play sand is usually very dirty, but it is a clean "dirty" so not harmful.
On to the method I suggest. I have tried changing substrate in the tank bit by bit but it was not easy. And you absolutely do not want to mix substrate materials. This will look hideous, believe me; the sand will filter down to the bottom quickly, leaving the gravel on top. You want to remove all the gravel, and have just sand.
I have changed the substrate in all my tanks ranging from a 10g up to a 115g, from gravel to sand, and some tanks more than once due to experiments or whatever, and I recommend a temporary tank [a spare 10g works fine, and worth having on hand for emergencies] filled with water siphoned from the existing tank (from the top so as not to get any detritus), removing the decor, heater and filter to this temp tank, netting over the fish. Cover the temp tank as fish do jump. This gives you adequate time to do the job properly without rushing. Drain the existing tank, scoop out all the gravel and dispose of it however you intend; it can go in a bucket and you can wash it later if you have another use for it.
Wash the play sand in small amounts in a pail until you can see the sand through the water. You'll never get all the dirt out, but that doesn't matter. Put the washed sand into the tank as you do each pail. When you have roughly 1.5 inches of sand level throughout the new tank, or 2 inches max, you're ready to add water. Place a large clean bowl or similar on the sand and run the fresh water into this; this stirs up very little sand. Add about 4-5 inches; I squirt in a small amount of dechlorinator, then arrange the hardscape (wood, rocks, decor, plants). This will likely stir up dirt, so I then siphon out the water down to the sand. Then fill the tank about 3/4 full, using the bowl again. Use dechlorinator at the start of the refill, and have the water just a tad warmer than the water in the temporary tank.
Providing the parameters between tap and tank are relatively similar, I would do a 50% water change on the temporary tank when you remove the decor. At this point you can get the tank running by moving over the filter and heater. Fill the tank close to the top. Move the fish over; sometimes I just net them from tank to tank, other times I use a pail and mix the waters half/half. The latter helps with very sensitive fish, or if parameters are not close.
Byron.