Hygrophila difformis is pretty hardy. If it does lose the lower leaves, you can just cut the tops again and replant. More likely, you will get a more healthy plant from it though. This plant can grow very fast when it wants to reach the light at the surface, but it will be very stemmy due to this fast growth. Replanting the tops encourages it to grow slower in height, maybe because it gathers more light with the bushier leaf?, I'm not sure. But the side shoots will be fuller and the plant will look much much better overall.
The bottom stems, once the tops are cut, probably won't ever look very good. New stems will branch off from it, but you'll always have that somewhat barren stem at the bottom. You can always let the bottom stems float on the surface though and eventually you will get some bushy growth from that to replant.
In this picture, my single wisteria plant is on the right behind the lotus. This plant started as a top I removed from an old tank. Even with the top completely shading the bottom of the plant it is growing side shoots at the base of the stem.
