Depends on the plant. Some stop growing. Some bend over to stay submerged. Others like Vals will keep growing and cover the top of the tank. Some if they can be grown emersed, will return to their emersed from.
The best thing to do is to keep them trimmed so they don't break the waters surface in most cases.
One advantage is with plants that break the surface, once they have, they are exposed to CO2 in the air which is a much higher amount than that of the water. It may actually give them a boost in growth.