"If it ain't broke, don't fix it" is good advice when it ain't broke, but if the dog is having ongoing diarhhea, something clearly IS broken here. Food can be a real irritant and the dog having eaten a given food "for years" is not evidence that the food isn't the problem. Many food allergies develop after years of exposure. Also, we know that manufacturers change recipes or ingredient sources and don't say a word to anyone. So the same brand and variety is not necessarily the same food that it was two months ago. It only takes ONE ingredient that disagrees to cause complete GI chaos.
If other issues (giardia and other parasites, organ issues and so on) do not explain the situation, then if a food change hasn't been tried, it is certainly appropriate to try that. Moving to a protein the dog hasn't had before and a carb source that's also different (such as sweet potato) is not only practical these days, it's easy to do. Dog foods.... really really tasty dog foods... made with novel/limited (fewer ingredients means fewer opportunities for allergic reactions)ingredients abound in every pet store. I'm not saying make fifteen food changes, but the OP's reply seems to indicate that hasn't been tried and if it hasn't, it's time to look at that.