
Hi David,
Sorry I can't remember what breed you have, but I'm guessing beagle! I would do what you are thinking, at 6 months gradually (over a couple of weeks) swop him over from puppy to junior food, and gradually cut down the size of his lunch and increase his breakfast and dinner accordingly. (I generally give my lot a Bonio for their lunch just to keep the wolf from the door!)
Good luck! :)