You start today by adding 2 ounces of whole vitamin D milk to her bottle then make the rest formula.
Then when she is taking that very well and her poop in normal again you start adding half milk and half formula. This is a stage where they often refuse it if they don't like milk. It's also the point where you might see a big change in her poop. It could be lighter in color and stink more.
Then when she's taking that well and her poop is more normal you go to 6 ounces milk and 2 ounces formula. This stage can be really short. If she is pooping normal and not constipated or having diarrhea or it's not stinking so bad you puke every time you change her then you can move on to whole milk.
It's often hard on their little systems when they go too fast and their poop is horrible. For you that is, it can stink so bad you'll have to change diapers with a trashcan right there so you can puke in it.
When she's taking whole milk in her bottle all the time you can start offering her a sippy cup. BUT if she needs to suck and you take the bottle away she'll find something else to suck...like her thumb or her fingers or a corner of a blanket. If a child needs to suck they will suck anything.
There is no biological need for a child to get off their bottle at any certain point. They can drink their milk from a cup, a bottle, a straw, any way they want.
Milk is milk is milk is milk. It does not matter if it comes in one container or any other. It does not matter if your child takes a bottle or a cup. They are still drinking milk. That does not make any difference to her teeth or her bite in her teeth. Drinking milk does not hurt the teeth.
What does cause tooth/milk rot is allowing a child to hold their bottle in their mouth all night with milk in it dripping on their teeth all night long. The milk never gets rinsed off with their saliva. So it sits on their teeth all night, their teeth are drenched in milk all night. This hurts the enamel. Drinking milk does not cause this damage, holding milk in their mouth all night does damage their teeth.
SO there is NO reason your child HAS to be off the bottle at 12 months of age. They may need their bottle longer. Just don't give them a bottle to drink all night long. Give them a bottle then take it away when they are asleep.
Tooth decay has more to do with genetics than if they took a bottle too long or didn't take care of their teeth right.