A.S.
I never changed nipple sizes. Both my kids used newborn nipples until they stopped using bottles @ about 14 months.
If you are nursing (vs. giving breastmilk in a bottle, it wasn't clear to me from your message), you may want to NOT make it easier to get milk from the bottle. Babies have to work a litle when they're nursing anyway, and it's possible that if you make bottle-feeding easier, they'll be less interested in breastfeeding.