Another reason not to switch is that iron is important for your baby's nutrition and health. Babies who don't get enough iron can become anemic (low blood count), which is thought to contribute to slower neurological development in children.
If your baby is having hard stools, it's fine to give him a small amount of water each day. One ounce of plain water, once or twice daily, should help to make the stools softer. Be sure not to give too much plain water, and if the problem persists you should talk to your baby's doctor or nurse, to discuss other remedies.