Let's assume some drug law is relaxed in the future. Do all prisoners currently serving sentences get released automatically? I don't think so, since they were convicted and sentenced under current law. I believe it would take an additional act of the legislature to release those prisoners. (This Supreme Court ruling seems to make this step unnecessary in the case of money+overturned conviction)

And let's not forget that they won't get anything for the years and opportunity lost while in prison for things that are no longer crimes.