Switching gradually between different quality foods can prevent stomach issues. Its not life threatening to do this to a healthy adult dog but since its a very young puppy (too young to be legally sold in many states) I suggest it. A 7 week old doesn't have much body weight or hydration to lose before getting really sick.

I would like to suggest Avoderm of the two foods you mentioned. The only things I don't like in their basic chicken food is avocado and alfalfa.

I consider Blue Buffalo to be sweetened poop pellets. There are two artificial sweetners in Blue, both saccharides. These can be bad for dogs because it messes with their blood sugar and why would they need artificial sweetners? Then the last 5 ingredients in Blue are scientifically named bacterias, including Enterococcus Faecium. This bacteria IS poop. Its a by-product often called "animal digest" unless the food company is trying to cover it up. Blue Buffalo is definately trying to cover this up. This is why they need the artificial sweetners.

Make sure if you don't know an ingredient you look it up on your own. Don't rely on potential high school dropouts working retail at the pet store. If they were experts they wouldn't be working a cash register for minimum wage.