Should Prisoners be set Free When Their "Crime" Becomes Legal?

[Dystopia]

Not necessarily.  Many countries once required drivers to use the left side of the road and now require them to use the right side.  Tax laws change continuously, but not necessarily for moral reasons.

IMO, that doesn't classify as a decriminalization/legalisation, it's merely changing the law. I'm talking about specific cases when we decide something becomes legal after being illegal. Tax laws usually change the amount etc but unless you legalise tax evasion there's no reason this applies here

And changing the law may not be because we find an activity now acceptable, but rather an acknowledgment that prohibiting the activity actually causes the practice of the activity to rise, or creates so many other negative consequences that we'd rather put up with the activity than suffer the side affects of making illegal.

I don't see any circumstance when this would apply - If something is wrong/harmful/immoral to the extent it is illegal then I don't see reasons to legalise the behaviour - The fact we can drop crime rates with weed legalisation is a nice bonus, but the main argument is that drugs are a health concern/individual choice and not a criminal activity (consumption at least)

There is no such principle under U.S. law.  Although, there is a principle that laws held unconstitutional may be repealed retroactively.  In which case violators would indeed be freed.  My personal opinion, not an opinion of was the law is, is that if we acknowledge that the law was immoral, a human rights violation, or unconstitutional, then people previously convicted should be freed.  In some cases they ought even to be compensated.  

Can you be sentenced for a crime that is not specifically on the State's law? How?

But many laws are merely judgment calls in gray moral areas.  I see no problem in retroactively forgiving people by law.  But merely repealing a law should not have that affect.

I agree.

I'm not sure the results of that line of reasoning would always be for the best.  After all, you can't comply with regulations until they are passed.

Why?