The only difference really is that follow on milk is often made with a higher proportion of a different type of milk protein, casein, as opposed to whey (I think first milks are usually almost entirely whey-based). Generally, casein can be less easily digested than whey protein (it's often used in hungry baby milks as well). We actually tried cow and gate follow on milk around 6 months on a whim (they were out of first milk powder) and in my experience, it cleared up my daughter's constipation, even though it's can be rougher to digest for many babies. She'd had very irregular poos since birth (even when BF), but it improved drastically on follow on milk and also once we were weaning. Purely for that reason, I decided to keep using it. So I think the only reason not to switch is if it causes tummy issues. If your LO has been responding well to first milk, I wouldn't necessarily be in a hurry to switch, but there's probably no harm either.