i would keep her on the formula, personally.
the reason why the soy can't be used as a main source of milk until after age 2 is because it is lacking in certain vitamins (can't recall off the top of my head which, perhaps vit D?) and fat.
by keeping her on the formula, you are ensuring she is still getting the needed fat and vitamins.
the other option would be to get lactose free cow's milk (it's called lactaid in canada but not sure in the UK) which is only missing the lactose, everything else is there, OR the lactaid drops, which you add to regular milk and it breaks down the lactose.
HTH