i think if your baby is okay with the current one, should be alright. but if he is drinking slower as compare with las time, then better change.
dear son only change to 3rd month teat when he was around 5mths old. and he took 3-4 days to get used to the new teat. he is now 6mth+, we have not change his teat to the 6mth one.. maybe next month or so.. =)