If the bag is unopened and you still have some of the old food left, I would mix what you have left and return the new bag. You could also donate it to a local shelter. A shelter will take an open bag so if you're completely out of the old food, open the bag, keep a week's worth and donate the rest.
I would gradually mix in the new food over a week or so. I'm not a fan of tippy toeying around food changes, the more often you change your dog's food and the more variety they have their tummies just get more accustomed to it. I rotate my dogs' food around so that they get a variety of nutrition. I can switch them cold turkey from one food to another and their poops don't suffer.
Keep in mind that Orijen is a very rich food and lots of pups get runny poos on it regardless. You might want to get yourself some canned pumpkin for the switch and add a little bit of it to each meal just as a preventative.