Dr Brown bottles

So I bought 6 dr Brown bottles before LO was born & just noticed the other day when I went to go use them there are different stages for preemie, nb, 3 mo, 6 mo, 9 mo & 12 mo. The bottles I bought are all stage 1 for newborn. 
Does anyone else use these bottles and do you NEED to really get the new bottles for every "stage"? I feel like it can't hurt her to keep using them.