personally, I wouldn't even bother changing the substrate. Your plants will grow fine in the pea gravel. IF you do go with heavy root feeders, just drop some root tabs in.
:iagree:
Putting the sand in the tank can't hurt if you want to do it. If you put it on top of the gravel, it will form a layer underneath that your plants' roots can grow in easily. If you change the substrate, it's going to be a little time consuming and a bit of a pita, but the plants might grow a little better.
Really, neither of these is necessary IMO. A root tab under heavy root feeding plants will serve you well. As with every tank, keep in mind that what will grow for you and what won't is really a lot of trial and error. Go slow and have fun!