In most of Europe, actually, laws can be retroactive... What is forbidden is a harsher criminal law. For example, creating an incrimination by law after a crime has been committed is a harsher criminal law and is forbidden. But if the legislator wishes to repeal a criminal law, the new softer law can be retroactive. It's called retroactivity in mitius.

I don't know if it's the same in Germany, but it's probable[1]. If so, then yes, the german legislator can make this case disapear completly by making the laws on the subject softer.

[1] Actually, I used to think that this rule was universal, because it seemed to be common sense... but that rule might not exist in countries of Common Law ?