It depends on individual Mine going 4mths still using slow flow teat (hegen) since born. If flow too fast just use back the old one but if flow is ok, try few days maybe not use to the flow. If milk flow around the mouth area or sucking rapidly meaning too fast.
I think it may take some time for your baby to get used to the new size and flow :) so try fir awhile more and see if it helps!
It will take a few times to get use to the difference if baby needs the changr.. but why the change since latching well for S?
Need to use a few time before baby get used to it.
Keep trying its just a matter of time...