Wow everyone! This topic got tons of opinions and replies! Thank you so much! I actually bought new food tonight. Won't start it until tomorrow though! I ended up getting Blue Buffalo - Wilderness. Puppy formula of course

I can't thank you enough for all the input! After reading that Purina isn't good, I felt I needed to go out right away and get something else.
Now my question is. I know you're supposed to slowly transition them into the new food. I read on the bag that 3-20 lbs, 3-5 months the recommended feeding amount is 1/3 - 1 1/4 cup per day. Gabby is 2 lbs 15 oz. How would you suggest slowly transitioning her? How much of the new food and for how long? And when do I know to start increasing it?