Transitioning off of alimentum formula!! Help!!

My DS is 10 months old, and his doc says it’s time to try to transition! What is the best way to transition off of similac alimentum formula? What formula did you choose to use? What way did you introduce new formula (I.e 1 full bottle of new per day, mix 75old/25new and gradually switch, cold turkey alimentum)??

My DS doctor said to try similac advance, one time a day, but he had diarrhea and vomiting on that, so we’re back strictly on alimentum. His ped said we can try any milk based formula. Just curious what everyone else has done!!