Ever had to go in for jury duty? And the lengthy selection process? All that stuff happens for a reason. Juries are specially selected to avoid a conflict in the case being tried -- hell, Capone's second cousin could have been in the jury next door for all they knew. That's why a judge doesn't have the power to place a jury that the lawyers haven't selected or interviewed, let alone do so mid-trial.
But this point becomes moot minutes later, when, seeing that the evidence is stacked against them, Capone's lawyer switches his plea to guilty. The court erupts, cheering, while Capone punches out his lawyer and is then seen in the background being led off to jail screaming, "Is this justice?"
Well, no, actually.
This is Sean Connery.
The whole point of the scene seems to be that Capone's lawyer switches sides and pulls the rug out from under Capone. But in real life, a court can't accept a plea without a defendant's verbal consent, no matter what the lawyer suddenly announces.
"Oh, and that DUI I got charged with last week? He's pleading guilty to that, too."