We have changed our daughters milk a few times now (from standard formula to prescription formula for cmpa, then to regular cows milk, then back onto prescription, then onto cows milk, then finally onto water) so I feel like a bit of an expert, lol.
We always make changes gradually, 1 oz every day or two. So say she is having 6oz bottles. We will then make 5oz normal formula, 1oz new milk for a few days (it depends how many bottles they are on a day - she hardly gets any bottles now so we do it over more days) - then we do 4oz/2oz, 3oz/3oz, 2oz/4oz, 1oz/5oz, then we stop using the old milk and only use the new one.