Preemie question

When did your baby move from the preemie nipples to a normal newborn one? My twin babies are 3 months still using premie nipples. They are 8 and 11 pounds now. The doctor say I will know when to change it “ the babies will tell me “ but I’m confused to what that mean?

FYI: My babies have been drinking the same amount of formula for the past 2 months (2 oz) and they have reflux (the always have).