The expectation would be at that point that Biden is asked to pardon (or commute the sentence of?) Assange. That's the political solution if the judge were to not accept the plea deal and remand Assange.

I wouldn't expect the judge not to go along with this though - he is pleading guilty and did serve what is now being called a sentence and presumably the US government can say that there are other benefits to his freedom that should not be overriden by the judiciary.