We just swapped formula out and replaced with cows milk for our son - that worked for us but I know others who’ve done a phased approach. We have saved a lot though - a bottle of milk is much cheaper than formula!
We added an oz every other day of cows milk to formula. Until all cows milk that worked for us. X
I started adding it to the formula slowly and then up the ratio. So 3oz formula 1oz cows milk for a week, then 2oz cows milk 2oz formula and so on until it was just cows milks.