As I am sure you are aware, the more you put the baby to the breast, the more breast milk you make. This has to do with your body's hormonal response to the nippling. Thus, if you begin to decrease the number of times breastfeeding, you naturally will begin to make less breast milk. The initial introduction of a bottle can also result in your infant being a bit fussier. Your infant may not want a bottle as often; part of the time you may simply be comforting the baby. Therefore, some mothers will alternate the type of feeding each day or by dropping out a breastfeeding or two every few days over several weeks, until their infant has fully accepted the bottle. This slower transition may also help make the feeling of engorgement in each of your breasts bearable.
You have not provided any information on why you are intending to switch to formula. I generally recommend breastfeeding for at least the first year of life or for as long as possible in the first year since breastfeeding has so many health benefits for you and your infant. I would encourage a discussion with your infant's doctor, if you haven't done so already. Perhaps an effective solution exists which will allow you to continue to breastfeed comfortably even some of the time.