[anonymous]

I would mix old with the new 1/2 and 1/2 for about a week, if all goes well then switch over completely to the new.
PS: The loose stools may be related to the antibiotics he was on, if it continues you may want to give him boiled rice and hamburger/bland diet for a couple of days till his stools firm up.

This is not veterinary advice; consult your veterinarian.