I think switching should be gradual and take a week. If his poop is that color, it must have been cleaning/flushing out the system. Sometime's they do get upset tummies when you switch to fast, but you can put some pumpkin in their food and help with the diarrhea/loose stools
i'm talking about real pumpkin, not like the pumpkin pie filling

it has some fibers in it that help with either diarrhea and constipation

but good choice with Innova!