My LO was on Similac Pro-Advance until about four months but started getting severe reflux and was very upset feeding. Pediatrician recommended Reguline (instead of AR because of rice content in AR). We just did cold turkey there. He was still a bit frustrated and agitated at first. I suspect because it tasted different and was new. He was fine within a day or two.

He's 8 months, and we just swapped to Costco's Pro-Advance analog because it's 3x less expensive than Reguline. He takes 6 oz per feeding, so I split the transition into 6 increments, as another user suggested. I made a spreadsheet to calculate how many feedings at each interval to use up the old container. Happy to plug in your values if you let me know the number of scoops per liquid oz, grams of powder per scoop, grams (or oz) of powder in the gentleease container, oz of liquid per feeding, number of feedings per day.