If I was arrested for marijuana possession 6 months before the law changed, is there anyway to reopen the case under the current laws?

Not likely.  You have two issues: whether your plea was knowing, intelligent and voluntarily and, to the extent that it was not, whether your offense occured after the effective date of the de-criminalization.  If your concern is the affect the charge may have on employment, etc., you may instead want to attempt to have the record sealed.  Best of luck.