I don't really remember switching the nipples - I used the ones the hospital gave me till they stopped using a bottle. When I was leaving the hospital they gave me same pre-made formula and single sealed nipples... we only used 2 bottles a day washing them after every use, which ment we only used 2 nipples. So, I stored the other ones still sealed till the open ones wore out and the opened 2 new ones.
I did try other nipples, just didn't like them. I think the switching should be based on your baby - not what the package says. If you switch them to early, it may cause them to chock on the formula. So, be careful when switching them.