I would think it's ok. I believe I read somewhere that it's ok for a kitten to eat cat food, but because they're still growing, they may eat more of the cat food to get the same amount of nutrients that they need that may be in a smaller portion of kitten food(because kitten food has more protein and other nutrients than cat food). Did that make sense? Of course, your vet could confirm this. But I really don't see a problem of switching at 6 months.