I don't agree with this statement. It is often best especially for a large breed to switch to an adult food at 6 months to slow down their growth so they don't reach their full adult height too quickly which can cause joint problems.
Leelaa17 you should speak to Jenna's breeder and see what they recommend regarding food type and when to change to adult. Most GSD breeders recommend Royal Canin GSD, Pro Plan, or Artemis.
In the old days there was only puppy food and adult and that's it, so they used to switch over to adult early. But now we have a wide variety of good puppy food for puppies of large breeds. These are designed to get the dog to grow slowly to avoid developmental effects in the skeleton of controlled amounts of calcium and energy(and something else but I can not remember the English word as my native language is Norwegian ), and will work better for this purpose than to use an adult. Depending on the size of the dog and kibbel brand the food will be used up to 12 and even up to 18 months before moving on to adult.
so Leelaa17 continue with the puppy food, good that you got something especially for you German shepherd puppy. Jenna will probably have little trouble with her stomach if you mix in too much of the new food immediately. give it some time, and if it does not work then it is maybe that she has a sensitive stomach, and check with your veterinarian he can give you more information