I did it this way, dk whether correct or not. Mine was Nan Supremepro > S26 > Nan Optipro. Eg: 1st bottle of the day - The one she usually takes (Nan Supremepro), then second bottle S26 (try out), if cannot then switch back to Nan. If can, then continue S26 for next feed. For mine, after S26 cmi, so I continue Nan supreme pro then try out Optipro the next day and switched to Optipro after that.