my baby is 9 months old. same case as SEAVL. tried to change a bigger size but she refused to drink. i have asked a consulant. as far as she could finish drinking the milk in about 10-15 minutes, no need to change to a bigger size. as changed to a bigger size just let she drinks faster. if the baby feels like there is a chance to "choke", she will refuse to drink...
i have stocked 6 bigger size at home... costover $200 (i used NUK)