It is possible to enter a Not Guilty plea and then hours before your actual trial at the Supreme Court gets underway, one changes one's plea to Guilty, then the trial is abandoned, and it goes straight to the judges sentencing. Summing up of the case and sentencing, we still get the summing up.
Of course, one gets a discount for saving the court time and money, even though it is a last minute exercise, it still gets in there for the discount. If you go to trial, pleading not guilty, and you are found guilty, you can appeal. But you get no discount on your sentence, which usually means , the years at which point you can apply for parole. Which does not mean you get parole, you are merely allowed to apply for it.
Everyone can appeal, there is no qualifiction for that. Not a lot of appeals get thru, though, there has to be new evidence, usually, or gross misjudgement by the prosecutor, or one's own barrister, or even a cranky judge.
So he could go quite a long ways yet on a Not Guilty position. Right up until the morning of.