[How Often Should I Change Protein Source in my Greyhounds food?]

I was feeding my dawg the same dry food with same type protein for about three years until I learned it should be changed regularly. How often should it be changed? 

Thanks in Advance ! 

[Are The Flavor Options Just Dog Reference?]

Grace prefers chicken to the fish variety of her kibble but I've found that she likes a spoonful of tinned pure pumpkin puree mixed in which has the added advantage that it helps firm up her stools.

 

Is canned 100% Pure Pumkin the same as puree? I give my dog a spoonful of non fat non flavored yogurt and have consider switching to the purée as well.

[Are The Flavor Options Just Dog Reference?]

I am looking to change my hounds food from 4Health Grain Free Large Breed to either Diamonds Natural or American Journey as i have hear good things on both. He has seemed to always skip multiple meals once a week or two and i was wanting to change to see if it helps. The 4Health is Turkey based, so i was thinking i would switch. Is there a flavor that most dogs seem to like better or is it each dog just different. I see there is Salmon, Beef, Chicken, and Lamb. Thanks in advance.