I think follow on milks are just a way of squeezing the last little bit of cash out of you as your little one moves on and doesn't need them any more.Yes there's the ad where the baby needs an enourmous cup to get all his daily needs whereas the formula provides it in just a small cup's worth.Of course you're supposed to think omg my baby needs this stuff.It doesn't go on to say that if your baby is getting a full and varied diet and a pint of cows milk besides there's no need to keep them on formula.Funny thing that,when the "advice "to keep your baby on formula is put out by formula manufacturers.It's no more useful than any other artificial vitamin supplement
Making the change can be tricky,the longer you leave it after a year the more savvy the child is and it'll be harder.I just keep the same bottles and change just an ounce a day,changing one more each day and going back if I think I've been rumbled,till we get to all cows milk.Merlin hasn't noticed the change we just made to 6 month follow on,he won't notice when we do it again in 6 months.All my kids drink milk twice a day still,some are bigger fans than others,but noone actually hates it,it's just a normal part of their diet.
If your baby has a poor diet or growth problem there's place for it maybe,but not in the vast majority of cases.