As long as he doesn't have tummy issues (growling, upset tummy or diarrhea, or maybe even constipation) it should be ok. Continue to offer the old food. If after another day or two you notice his eating the new food is not causing any issues, stop the old food and stick with the new.
Just in case, if he does have tummy issues, you can give him probiotics or canned pumpkin. Careful not to pick up any canned pumpkin pie filling.
Either item is good to keep on hand anyway.