I was railroaded into taking a plea bargain of a crime I did not commit.
Criminal Defense Attorney
Sorry, there's no such process to change a plea you entered thirty years ago.
Entering a plea is like getting a tattoo; you had better plan on living with it for the rest of your life.
There are procedures to withdraw a plea before sentencing, or to challenge a plea through a petition for writ of habeas corpus while the defendant is still in jail or prison, or on probation. Even in these situations, courts reject the request and the plea stands.
In a very unusual case, where the alleged victim comes forward years later and admits lying, the courts might consider hearing a writ petition to overturn the petition. You'd need a very experienced and skilled attorney to make this fly.
Please understand that this is a general discussion of legal principles by a California lawyer and does not create an attorney/client relationship. It's impossible to give detailed, accurate advice based on a few sentences on a website (and you shouldn't provide too much specific information about your legal matter on a public forum like this, anyway). You should always seek advice from an attorney licensed in your jurisdiction who can give you an informed opinion after reviewing all of the relevant information.