You don't need to switch to grower then layer. You can feed a good quality starter formula for life then offer oyster shell on the side for the layers.Do you have a brand you recommend?
When the hens start molting in their second winter, they do not need all the calcium found in layer formula and will need more protein, which the layer formula does not provide. It is also very convenient to have an 'all flock' type of feed when you have chicks or roosters as they can also be harmed by eating excess levels of calcium.