The decision on whether to agree to Polanski's request lies with Judge Espinoza. He says he'll hold a hearing on it January 22.

Also from the Court of Appeals decision, quoting Judge Espinoza at the earlier hearing:

“[H]aving reviewed all of the evidence in this case, notwithstanding the People's assertion that the misconduct that occurred is still in dispute, there was substantial, it seems to me, misconduct that occurred during the pendency of the case which will be among the many factors that would be considered by me and any other court that would sentence Mr. Polanski. He had a plea agreement with Judge Rittenband. Unfortunately, Judge Rittenband is long since deceased, but the terms and conditions of that plea agreement are well known.”

And on what Judge Espinoza didn't consider:

While Judge Espinoza has expressed the view that Polanski is required by section 977 and the bench warrant to be present at any proceeding regarding his case, on this record it does not appear that he has ever been asked to release Polanski from that obligation, as he is authorized to do by section 1193.

If Judge Espinoza grants Polanski's request, and holds an evidentiary hearing and finds misconduct, the case against Polanski could be thrown out entirely. The appeals court said:

The trial court is fully empowered, upon examining the evidence in this matter, to order an evidentiary hearing on whether to dismiss the prosecution in furtherance of justice. Such a hearing would permit the trial court to make factual findings as to the events in 1977 and 1978 and to dismiss the prosecution in its entirety if the facts so warrant.

If Judge Espinoza doesn't throw the whole thing out, a sentence to time served is the most likely alternative. As the Court of Appeals said:

If, after taking evidence, the trial court finds that Polanski’s allegations are true and that the original trial judge agreed that the prison stay for the diagnostic study would constitute Polanski’s entire punishment, a condition Polanski fulfilled, the trial court could find that justice requires that the trial
court’s commitment be honored and that Polanski should be sentenced to time served.

...we are confident that the trial court could fashion a legal sentence that results in no further incarceration for Polanski.

Looks like things are finally looking up for Roman.