In general, society’s approach is that if you broke the law at the time, you’re a criminal, and future law changes don’t change that. So you would stay in prison.

I’m not sure there are too many real-world examples of that, but there definitely are examples of people not in prison but with a criminal record for something that’s no longer illegal. An example that jumps to mind is that it took until 2017 for gay men in the UK to be given the right to apply for a pardon if they had been convicted when homosexuality was illegal. Note apply for a pardon - it’s not automatic.

None of them were actually in prison, but many felt that having the conviction on their record was still a blight on their life.

This is a relatively unusual case because there was so much political and societal shame about the way gay men were treated and the unjustness of the law. That’s not the case with all or even most changes to laws, and I can’t think of another similar example.