You've asked a number of good questions. I can give my opinion, which is certainly not fact, just my experinces.
Orijen is a very high quality food. As a matter of fact I like it so much that I use the 6 fresh fish formula as training treats for my Pekingese.
How often should you switch pet food? That depends on a number of factors. If you are feeding a low quality food then you should switch your dog off of it quickly. If your dog is showing signs of allergies then you should switch. However, if the food you are feeding is good quality and you are having no problems with it then I see no reason to switch for at least 3 months, and I personally would probably wait 6 months.
As far as the best quality food, that's impossible to say, because each dog is a little different. Personally, I feed Instinct Grain Free. I recommend lots of brands as being equally good in my mind, Orijen, Taste of the Wild and Acana Grain Free being a few that come to mind. And of course, there is raw food, and homecooked meals to consider. So really the perfect food is based on what your dog likes to eat and thrives on, and what you invest your time and money in.
If you are dead set on making a switch, you can get a sample bag size of the Orijen and do what I do with it, use it as training treats. Then in 3-6 months when your dogs stomach is ready to switch foods again, it will be an easy transition for you. Also, when I transition dogs I use a probiotic, as I find it helps with the gas and stools, but that is just me.
