Switching Formula

I'm going to be switching formulas from a regular to a lactose free due to some health issues to see if helps my baby. It slipped my mind to ask the doctor this but what is the best way to transition from one formula to the next? Especially since the old one is Similac Advanced and the new one is Baby's Only Organic Lactorelief. I'm going from lactose to no lactose. So I don't exactly know if I should mix the two to make the transition easier or if mixing them would be bad because one has lactose while the other doesn't.