Switching formula?

Hi mommas! I'm in the process of trying to switch my 3 month old son over to nutramigen formula due to what we believe is acid reflux. I want to switch him and see if it helps him at all. Right now he's on similac sensitive, is it safe to be doing one scoop of each until he gets more used to it? Or should I not be doing that? I've noticed he's been pooping alot and I'm thinking that could be the cause? Any advice on this would be appreciated! Thanks!