Legal Question in Criminal Law in California

The case is over, unless the person wishes to withdraw a guilty plea and go to trial, and this must be done within a limited time.

No. Sentencing is the end of the case. Why would you think you can go back and get a 'do over' ? These aren't kids' games. You already raised and used all your arguments, or were supposed to, in order to get to the plea bargain you negotiated with the DA, or in your defense at trial. It it was a trial, you could timely appeal the case if there are valid legal grounds to do so, but few appeals succeed, and all are expensive.