It really doesn't matter whether the reductions occurred before or after probation. A 17(b) motion makes a felony a misdemeanor for all purposes. That means you have none of the disabilities that would attach to a felony conviction. Under CA law, It is unlawful for anyone convicted of a felony, or who is a drug addict, present or former mental patient, ever committed for mental observation, or acquitted by reason of insanity to own or possess any firearm.
However, people with certain misdemeanor convictions involving force or violence may not possess or own any firearm within 10 years of the conviction. In your case your 245(a)(1), even if reduced to a misdemeanor, would prohibit you from owning or possessing a firearm.
It does not matter whether your cases is dropped to a misdemeanor before or after probation. Based on the limited information you have presented, if the incident involved anything related to domestic violence you would lose your ability to own, use or possess a gun and bullets for 10 years. I would have to look at the circumstances of the case to give you a definitive answer on the rest of your constitutional rights.