I agree in general with the idea that retrospective law is bad law.
In this case, if the only change is making directors responsible as well as corporate entities, then I would have no problem prosecuting directors personally for such acts knowingly carried out or authorised by them or by someone under their reasonable control while they were in charge. If the law contains no change that prohibits additional acts, then I have no problem at all as I wouldn't consider that to be “retrospective”. I'm assuming that the illegal act was illegal at the time it was carried out, it did not become so later. Hope that makes sense.