What would the legal basis for this be? If you're on the freeway and get arrested for doing 75 in a 65 zone and next year that speed limit is bumped up to 75, you don't get the fine returned and the conviction expunged or deleted; the fact remains that the act was illegal at the time it was committed.
The speeding ticket would be valid. The felony charge of being found with a weapon in the vehicle should be invalidated. They should be able to clear that and get reimbursed. Maybe all those 'appeals' would break the state's coffers.
Look at it like those cases where an offical screwed up DNA samples in a lab and put all cases under his/her purview in question.
I'd try to be first in line if my rights had been violated like that.