First, make sure your little one is comfortable with a bottle and has no digestive issues with formula. Switching before you've established an alternative would be disastrous.
If possible, wean one feeding one week at a time. That gives you and the little one both time to adjust to the changes, but it does take time.
Once that hurdle is cleared, from what I understand (not for experience - from listening to others) there are medications available to dry things up quickly. Or if you prefer a natural alternative, believe it or not, put cabbage leaves in your bra. Sounds weird, but works - the cabbage leaves emit a chemical that prompts the mammary glands to slow production. Again, this is something I know to be effective from the experiences of friends, not from personal experience. But I've read about it, and it is well documented, and not just in the loony sources.