When you first bring a new pup home it is very important to continue to feed the same food the breeder (or shelter, etc.) has been feeding the pup for at least 2 weeks. Rule of thumb is to go thru 1 bag and as you get near the end of the bag start your switch over gradually.
Pups are under stress when coming to a new home and changing the food can add more stress which will most likely result in loose stools or severe diarrhea.
As mentioned above; when switching do it gradually to avoid loose stools. Slowly substitute the new food for the old. ie; if you are feeding 1 cup per meal substitute 1/8 to 1/4 cup of the new food for the old.
Wait a few days and observe stools. If they are firm than continue your slow switch over.
Keep in mind that loose stools can also be caused by parasites, coccidia, giardia, etc. so if your pup does not firm up take a stool sample to the vet to be sure there are no medical issues.
You should always take a stool sample with you to your pups first visit with your vet.
Some people believe in rotating foods. I don’t feel it is necessary and certainly would not do this with a young pup.