It is, as I understand it, retroactive in that sense. For instance, in the case of live-in caregivers—and perhaps someone can correct me if I'm wrong—the age of the child was locked in at the date when the live-in caregiver first applied for a work permit to come to Canada. If they were dependants at that time, then they would forever be dependants. In that sense, it is retroactive, and I think that it can be easily managed by instructions or a simple change in the policy.
On October 20th, 2016. See this statement in context.