system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:When a criminal offence becomes legal, what happens to the prisoners who were convicted of it?

Example: What would happen to all those people who were imprisoned for drug possession/sale if/when weed becomes legal?
gpt
In most jurisdictions, they would continue to serve out their sentence unless the executive branch decided to explicitly offer pardons. The idea that *changes in law don't take effect backwards in time* (i.e. changes are not retroactive) is generally an important principle of law and is a constitutional requirement in some places. This also means that you can't be charged with a crime if the act wasn't a crime at the time you committed it.