A substantive criminal matter commences when an information is laid before the court. That's the formal instance under section 504 of the Criminal Code, when a proceeding begins. That's your test for whether or not the transitional is triggered. If an information has already been laid before the court in order to commence proceedings, it wouldn't apply. If that has yet to happen, it would.
On May 10th, 2023. See this statement in context.