First the reduction can occur only if you were convicted of a felony where the law you violated was a wobbler (capable of being charged as a misdemeanor or a felony). Not all crimes are reducible; in fact most aren't. Not being guilty is not a ground for reduction.
Writing the judge will neither help nor hirt as the judges do not read mail from defendants. If you have to write someone write your lawyer.
Now you feel like the judge needs to hear the whole truth? The judge based his or her acceptance of the plea bargain on what you stated in court about your case. The best time to contest the truth of the charges is before you accept a plea deal. If new evidence is now available, or you were not properly advised by your attorney about your constitutional rights before you entered your plea, you may be able to withdraw the plea.
That is a difficult task. The risk and hardship of going through a jury trial often are enough to pressure people into accepting plea bargains, but at this stage you have already accepted an offer and the judge approved the deal.
The judge can legally reduce the charge to a misdemeanor only if the crime is a "wobbler", which are designated in just a small percentage of felonies. Some thefts fall into this category. Rather than venting to the judge, I suggest another path: talk to an attorney.
Since most felony cases in Sacramento come with five years of formal probation, after completing house arrest, you probably haven't completed enough of your probation for a judge to let you off the hook at this stage.
You should contact the attorney who represented you, or get a consultation from several criminal defense attorneys to more thoroughly discuss the facts of your situation. Good luck.