using a gravel type substrate underneath sand is just going to result in the sand eventually sifting down and the gravel substrate coming to the top. it won't stay the way you put it for very long.
i use play sand in my planted tank and the plants do fine. i don't dose ferts or run co2, so there is not a huge amount of lush growth, but the plants do grow and seem to be 'happy' enough. pool filter sand, with some root tabs tucked in by plant roots, would work just fine for you.
if this tank is well established, i'd change over the substrate slowly. there will be a lot of beneficial bacteria in your current substrate, and removing it all at once could send your tank into a mini cycle. i personally would remove 1/3 of the gravel at most, replace with sand, and then do additional 1/3 switches spaced at least 5 days apart, if not a week. replant your plants as you switch out substrate, or just let them float for a bit if they are a hardy enough species to do so (swords, anubias, vals, etc will be just fine floating for a week, i've done so myself).