Milk Review Suggestion

From the day my baby was born, we used to feed him Nan Opti Pro 0. And after a month we decided to feed him with S-26 Gold, but for few days, it seems na he always gets constipated. We bought 1 big pack, and we're on the first bag for now, is there a time for the baby to adjust his digestion sa new formula milk(bigyan pa ng time to fully matanggap and ma process yung s26) , or we should stop feeding him yung s26, and we go back to Nan? Thanks in advance for your feedback!