Are you still on probation? You might be able to file a petition for writ of habeas corpus if you were not advised of alternatives that would help you avoid a conviction.

If it was your first offense for simple possession of a drug, you probably should have been eligible for diversion under Penal Code §1000, which would allow you to avoid a conviction altogether if you completed a drug education program and stayed out of trouble for 18 months to three years. You may also have been eligible for Penal Code §1210/Proposition 36, which allows for dismissal of the case after you complete probation.

You should consult with a criminal defense attorney in your area to see what remedies might be available to you, based on the specific circumstances of your case.

Really? So would every other defendant that discovers the real consequences of crime and conviction.

'Change of plea' is possible prior to sentencing, but risky because you then would have to go to trial, no other plea bargain deal would be offered, unless you would have been eligible for mandated alternative sentencing. If convicted at trial, you'd be sentenced to the full term on all charges. That's what you were trying to avoid by entering the plea bargain.

After sentencing, you have to have grounds similar to an appeal, and a successful outcome is certainly not going to assured. I have to assume the judge would have disclosed any mandatory alternative sentencing that actually was available at the time, so therefore you will have to prove otherwise.

If really serious about pursuing this, if you qualify, understanding the risks and expenses, feel free to contact me.