Mine wasn’t drinking enough and wasn’t ready for the size up so the dr said to take a needle and make the current size bigger. Keep the bottle leveled so not too much milk is going in, and to take breaks every couple of seconds at first so they get used to the flow amount and are not trying to suck so hard. After about 2 weeks I changed the nipple size and did the same process of bottle level etc. she drools and spits up too but she’s still drinking way more than she was before I changed it. (She wasn’t drinking enough)
The age limits are just guides. Babies are all different, and different sizes. I wouldn’t size up just based on age guidance, wait until your baby is showing signs they are ready - eg. Getting frustrated/bored
My son was the opposite and he spit up a ton until I gave him a bigger nipple.
keep smaller size