They could vote to ease punishment for past offenders, but that would be an exception and not the rule. In general people are beholden to the law at the time they're caught breaking it. I got a speeding ticket, and a few months later the speed was changed to match the speed I had been doing. I still was guilty of breaking the law at the moment I was stopped.
So they could put in an exception for past users, but the law defaults to what the books said at the time you committed the crime. If that wasn't the case, people could be convicted of crimes that were not illegal at the time they committed them.
I have a side tracked question about this:
Let's say you were in prison for dealing Marijuana. What would the status of your case be now considering that it would be legalized?
(as in, would you still have to serve your term even though the crime you did is now considered legal?)