What if marijuana were legalized? What about people already in prison or jail?

If marijuana were to become legal (personally I don't smoke but can see the financial positive in the legalization), what would happen to people who were serving lengthy sentences for smoking or selling it or both?? It reminds me of the days during prohibition. Under the Volstead Act, alcohol was made illegal, but what happened after the Amendment was repealed? Were the bootleggers released if that happened to be the only thing they were incarcerated for? I understand ex post facto laws etc, but this is opposite. Suppose it really was illegal when you committed the said act and then the said act was legalized. For the overcrowding in prisons, I can see where the issue of marijuana would be a plus in making it legal. However, for my own intents and purposes...i hate the smell the stigma that is on it, and I don't want anything around my kids that can give them a contact buzz.