diarrhea after food change

Hi again. When I first got Max he was on Natural Choice Lamb & Rice and
that consistently produced "soft serve". I then talked with a pet store
owner and he turned me on to his own pet food called - In Pet's Choice, with
a primary ingredient of chicken meal. Max loved it and everything
"tightened" up. Now, I learn that he will no longer be making his dog food
and told me Diamond would be the closest replacement. I purchased Diamond
Premium Adult (I checked the date and with Diamond regarding the bag I was
to purchase). I have been mixing half and half for the last several days.
Max has only gone number 2 between feedings, twice a day (he eats in the
morning, goes in afternoon, goes again in evening and eats again in evening)
consistently. Since the food switch the second number 2 is always soft
serve with the first being solid. I don't get it. I know "All commercial
dog food is garbage" but if anyone has any suggestions I would appreciate
it - Here is the first couple ingredients for both -

In Pet's Choice (old food) - chicken meal, brewers rice, brown rice, chicken
fat, dried beet pulp, fish meal, brewers dried yeast, flaxseed, dried egg
product, etc etc.

Diamond Premium Adult (new food) - chicken by-product meal, whole grain
ground corn, wheat flour, chicken fat, brewers rice, beet pulp, fish meal,
egg product, flaxseed, natural chicken flavor, brewers dried yeast, etc.
etc.

shore...

Ingredients are an important part of the story but so are
nutrients and nutritional content, and without doing any
research I don't know whether or not the two feeds really
are comparable beyond the ingredients list. Among the first
things I'd check is fat content and whether or not the new
food has markedly more fat in it.

The standard home remedy for loose poops is canned pumpkin
(no seasonings added). But if the poops don't firm up and
really are pretty loose, switch foods.

And on a side note - anyone have any tricks or tips on cleaning up the soft
serve? He does it in the public tennis court of all places and I have a
tough time getting into a plastic bag with a stick. Gross I know, but what
am I to do !?!