I have no experience of FF but I switched each feed with breastfeeding. By that time though we were down to two feeds and so I switched the morning feed and presumably my supply dropped a bit because he started to drop the night feed. That said, at 17 months I have now introduced an afternoon cup of milk back in because of hunger.
Just give it a go, you might be surprised. Switching from a bottle or cup of formula to a cup of milk I reckon is a smaller jump than going from breastmilk but you might find she's absolutely fine with it. If she's not you can go back and try another strategy like mixing them and gradually increasing the amount of cow's milk.
I have heard loads of people say their LOs had more milk when it was formula but it's advised (in the UK anyway) to switch to a cup around this age and most people combine the changes. Lots of babies I know will have milk from a bottle (or the boob) because it's comforting as much as because they're hungry. That said, my son will drain a cup every morning and most evenings now and if you are worried about the milk intake, remember at 1 they need 1 pint or the equivalent in food so you can give yoghurt, cheese etc and you soon get there.
