I can't speak to all the legalities of it. Post-June 30, as the legislation is currently written, for a firearms business that would still be in possession of these firearms, the firearms would be deemed to be prohibited. The businesses themselves, if they wish to retain them in their inventory, would have to have the prescribed purpose of the prohibited firearms on their firearms licence. They wouldn't necessarily have to destroy them, assuming they have the prescribed purpose under the firearms licence regulations to continue to possess them.
On June 5th, 2018. See this statement in context.