you don't have to change, if your dog is doing well on it then stick with it unless your looking at the financial benefit of saving some money, Acana is still a good food.
There are myths about it being harder on the liver and kidneys but recent studys have shown this to be false if you are dealing with a good source of protein meat based there should be no issue , if you are dealing with a protein source like corn , soy, any by products or blood meal ect then it puts a bit more strain on the liver and kidneys to filter.
When feeding a high protein meal though make sure you are giving adequate amounts of water, this makes it easier on their systems to filter . alot of dogs will not drink enough on their own so best to suppliment their water source by adding it directly to their kibble . not every dog working dog or not will thrive on a high protein diet just as you can have a lazy house dog thrive on one. each is individual.
I switched to acana with my bunch when my boy was growing too fast , the vet wanted a lower protein it also saved us a bit since at one time with pups in the house we were sitting at 18 dogs.. the only other diet I would recommend over orijen or acana is to do a prey model raw diet.
Pit bulls are..... Beauty without Vanity, Strength without Insolence, Courage without Ferocity, and all the Virtues of Man, without his Vices.""