I agree with what Jazmine said. At my boys two month appointment she suggested starting to add half a teaspoon to every 3 ounces of pumped breastmilk for extra calories. He wasn't gaining fast enough in my opinion so I started doing bottles half BM/half formula. Pitcher method for both BM & formula. Now he gets bottles of just formula majority of the time as I am just not making as much milk as I used to. But I had no issues when mixing it with BM at first then doing formula only. I think you just wanna make sure that baby doesn't get constipated and get a little tummyache from such as sudden change so if you're able to do a sort of transition as described, I think that's the best bet it sounds like you've got some time, so I suggest trying to do that if possible!
i would say continue to pump when you can and mix the breast milk with the formula using less and less until you’re only using formula.