At Thrive we make our foods for all ages. We don’t feed our grandparents a different food and we don’t need to feed older pets a different food. Great nutrition works for us all. We do add some extra sunflower oil into our kitten foods as they need that extra fat as they grow, but other than that it is the same food. It does no harm to transition your cat to Thrive over a few days especially if Thrive is very different to what they have been eating before.