There's a distinction to be drawn between retroactivity and retrospectivity. I don't want to get too technical, but a retroactive application would change the substantive law. It would attach new legal consequences to something that's already happened in the past. In a sense, it would be changing the law as it was in the past. The minister stated during his speech that this is not permissible under the Constitution.
Instead, what we're doing is we're attaching new legal consequences in the future for events that occurred in the past, and these changes are procedural. I don't have the case law with me, but there is case law stating that this type of change, what we're calling a retrospective change, will pass constitutional muster.