The heart is essentially a muscle. Like any normal muscle if it has to pump against an increased pressure it will get thicker or hypertrophied. When that increased pressure is relieved to a great extent by valve replacement, the muscle will likely go back to a more reasonable size and thickness. If the heart is otherwise healthy, then a low salt diet and exercise will likely be all you need. Bring up these issues with your cardiologist, who can better relate these thoughts directly to your situation.