Certain offenses in California are punishable either as a felony or a misdemeanor. Those offenses are called "wobblers." If you were convicted of a wobbler as a felony, the judge may be permitted to later reduce the offense to a misdemeanor so long as your sentence did not include a prison term.
The easiest way to reduce a felony wobbler to a misdemeanor is to request it with your Expungement. Alternatively, you can file a motion to reduce any wobblers using the forms and instructions(PDF, 582KB) provided by the San Diego County Law Library.
Reductions via Proposition 64
With Proposition 64, people convicted of a marijuana related offenses may be eligible for a reduction or in some circumstances a dismissal.