There is nothing wrong with asking advice. Eukanuba may have been a good food once, I don't know, but it isn't the best at this point. Evenso, your puppy will be making quite a transition when he first arrives. Having food be familiar might avoid some minor issues for the first week or so. Then go ahead and switch him. And yes, it is best to do it gradually.
Your breeder gives your puppy a start, but your pup's health is up to you. And you are the one that is going to have to manage health concerns down the line. It does matter what we put into our dogs, that they will be eating, for the most part 90-100% of their diet. And puppies grow an incredible amount this first year. They go from about 14-24 ounces to 70-80 pounds in the first year -- boys do. That's a LOT of growing. When you think of a human baby that incubates for 9 months is about 6-9 pounds at birth and how much to they gain that first year? It isn't 50-80 pounds.
So yes, you're concern is real. Feed him the best food you can afford for this first year or two. Giving him a week or two to get acclimated is fine, then I would go ahead and switch because I think you will feel better about it.
Heidi Ho, Odie
Joy-Joy, Bear Cub, Hepsi-Pepsi
Cujo2, Karma Chameleon
Ramona the Pest, Kojak -- who loves you baby?
Tiny Tinnie, Susie's Uzzi, Kaiah -- The Baby Monster.