The eggs you sat a week ago will hatch out several days before the new one. The older chicks will have a big advantage over the newly hatched chick/s, unless you have the means for seperate brooders.
You also run the risk of the temp fluctuations, from opening the incubator (putting in new egg, candling, lock-down, and then removing the hatched chicks!) damaging ALL of the eggs.
You also run the risk of the temp fluctuations, from opening the incubator (putting in new egg, candling, lock-down, and then removing the hatched chicks!) damaging ALL of the eggs.