Please tell me, we're on a mixed feeding - I feed my son 4 times a day…

Please tell me, we're on a mixed feeding - I feed my son 4 times a day with a mixture of 30 ml, the doctor advised us to switch to another formula, lactose-free, how to transition to it, should we give it immediately or gradually? Once a day, other 3 times our usual mixture. How is this done? And who has tried lactose-free?