D.H.
It is always best to transition slowly. Especailly to reduce the pain you will be in if you just switch her. If she hasn't had many bottles, you will want to start pumping and giving bottles, then start adding 50/50 or so breast milk and formula to introduce the formula. Generally it is best to start the bottles at the happiest points of the day, not when its nap time or bedtime. When you know she isn't going to have a reaction to formula, then make those bottles full formula. After that just start increasing the bottles and reducing the nursing until you have completely made the switch. I'd say with in a couple weeks you should be pretty well done. Moving slowly will allow your body to slowly adjust to the reduced demand for milk, so when you stop completely there won't be pain from unneeded milk. It's amazing how your body will adjust. Good luck.