Legal Question in Employment Law in California

Since you work for a county I assume you have union coverage. Go to them and find out if the employer's actions are illegal [they likely are] and file a grievance demanding the promotion, difference in pay and benefits, seniority lost in the position, etc. Point out to maangement that if the court system is able to forgive your errors of judgment that happened 9 years ago why can not your employer rate you on your performance on the job; would they refuse to hire Richard Nixon, JFK's father, etc. Do not rely upon the Union to be aggressive, you need to push them.

Records are forever. Expungement does not 'remove' the conviction, but merely changes the record to show 'conviction reversed and dismissed by expungement'. The Labor Code bars employers from asking about arrests without conviction, but convictions can be 'legally' used in making employment decisions. If your union can protect you, so be it.