Changing Nipples

JAL102129

Ds is 5 weeks and has been eating very slowly out of the slow flow nipples for the past week or so. We bought some medium flow nipples today and the first feeding with them he didn't have any trouble with them except that he spit up a little more than usual at the end of the bottle. It wasn't much more but I was wondering if it was because we switched too fast. Does anyone have any opinions? When did your LO switch to the medium flow? TIA!