[cypher]

It depends a bit on how different the new food is to what your ham has been fed previously but you need to do it slowly, if it's not too different you could probably do it in a week if it's very different then a couple of weeks.
Begin by giving the food your ham is used to with just a tiny bit of the new food added then after a couple of days give a bit more of the new & less of the other gradually increasing until you're just giving the new mix.