A friend sent me this:
"When there is a plea bargain, the court can reject the agreement but it cannot change the terms of the agreement unless the parties agree. (People v. Segura (2008) 44 Cal.4th 921, 931; People v. Superior Court (Gifford) (1997) 53 Cal.App.4th 1333.)"
I haven't read the cases and don't intend to (too little time). Someone will. They postdate Polanski's situation by 20 and 30 years but if this is what they say I suspect they reflect the law in the 70s as well.
I'm sure any criminal judge knew the law. Maybe all the judge was really going to say was "no deal, try the case, or try another deal." In that case, no foul to Polanski--he ran because he didn't want to risk a conviction. Maybe the judge WAS going to enforce the guilty plea but change the sentence in some obviously improper fashion. In that case, I believe Polanski would have gotten QUICK relief. There are vehicles for the system to act speedily.
The legal system is thorny, and it can make mistakes. Judges can be dumb or corrupt. But Polanski started this and he can't run when it's not working out and then say, "too late, I skate." Come back. You can't get relief as a fugitive.