I agree that perhap's just emptying the tank and starting new would be easiest for relatively small tank.
I WOULD however leave a bit of mulm in the bottom of the tank to help kick start the bacterial process in the substrate.
Some folk's use ladies nylon filled with sand to help create/maintain slopes in the aquarium where everything seem's to want to settle over time, and slopes level off.
As for the flourite,,it is little more than clay product which help's hold nutrient's, but nutrient's take time to settle in substrate so this type of material work's better as it get's older.(plain unscented cat litter work's as well).
To add substrate to existing tank's you can rinse the substrate well and then place it in freezer bag's .
Then lower the freezer bag into the aquarium near the bottom,and gently pour it out on top of existing substrate.This method help's prevent too much clouding of the water.
If you go with soil,,I would remove the existing substrate,leave some mulm in the bottom,place a layer of damp soil about one and a half inches deep,then plant the plant's,then add about an inch of flourite,then cap it with about an inch or inch and a half of sand or fine gravel.
This way you don't kick up a lot of dirt trying to plant after all layer's are placed.
Take note to place plant's that will grow tall near the back and shorter growing plant's near the front.
With soil,,you don't wanna be moving plant's about lest you also end up with cloudy muddy affair.
hope some of this help's.
P.S I found that "Special Kitty cat litter" found at walmart work's just as well as flourite and is way less expensive.