The Shaggy Dog (1994)

Preteen Moochie Daniels just wants a dog, but his dad, Daniel, is allergic to canines, like Bundles, the shabby sheep dog of neighbor Charlie Mulvihill who secretly trained his pet to help him steal jewels. Mooch's big brother Wilbert 'Wilby' is smart, shy and a promising inventor, but hopelessly clumsy when it comes to girls, and is jealous of his slick mate Trey who has no problems. Desperate Wilby resorts to a spell from the ring of Lucrezia Borgia (he accidentally got from dad's museum of curiosities), which magically turns him into Bundles and back at uncontrollable times. This is how he also knows that the diamond on loan in his father's museum is Charlie's next target.