To some extent you can, the longer it ferments the more lactose is converted. I've read that anything over 24 hours will remove most if not all of the lactose from the yogurt. Anything you buy in a store will be cultured just long enough to be brought to market, in other words it'll have lots of lactose. It also has little or no probiotic value, even the ones that say "active cultures". When you make your own yogurt and you'll never eat commercial yogurt again.