Because the needs for a puppy differ from that of an adult dog I personally wouldn't be comfortable feeding them puppy food. You are risking at the very least weight gain from the puppy food, plus as someone mentioned there can be a big chance for diarrhea by switching abruptly from one to the other - even if it is the same brand.
Hard to believe that any reputable pet store would not accept the return of any unopened bags of food. I'm not saying yours isn't reputable; I know that even vet clinics will accept return of their food if the bag is open and your dog won't eat it. I'm like Kodee, they would have had no doubt that I wasn't happy without the exchange and that they wouldn't see me there again - but again, that's just the way I am! (Difficult customer - beware!! )
When all else fails, call your vet and get his/her opinion. They or a nutritionist on staff should be able to tell you what to do.
RIP Wallis, John Robert, Tibby & Pip
Mom to JoJo, Roxie & Linguini (Cavaliers)
and my birds Moji (Senegal) and Walter (Blue Front Amazon)