When I was still latching my baby, I tried not to upgrade the teats so soon so that my baby wouldn't prefer the bottle. From the NB Avent teat used at first, I upgraded to the 1+month teat at about 2 mths of age. I upgraded it because the NB teat was too slow and baby was getting too much air in his tummy. But from the 1+month teat to the 3+month teat, I only changed to that when he was 5 mths.
During change of teat, the sudden change in the flow might gag baby. So what I did was interchange the use of teats. When he was sleepy, I used the older teats. When he was more awake, I used the new teats. After about a week of doing that, he was accustomed to the new flow then I switch completely.