By itself, nothing, since they still broke the law when they committed their initial offense and were duly convicted of it. In principle, you could even prosecute and convict somebody after the legalization for an act committed before, though it seems unlikely any prosecutor would bother. Though if you bothered to legalize something, you might also want to commute the sentences of anyone previously convicted.

This is a mirror of "Ex Post Facto" - if you pass a law making some act illegal, you can't convict somebody who did that act before the law was passed because it wasn't illegal at the time. In principle, it's supposed to be possible for an individual to determine if an act they are considering performing is illegal or not, and for them to be able to rely on that determination.