Lawfag sr., here. A judge would pitch a fit if a prosecutor reneged on an immunity deal, assuming the defendant was acting in good faith pursuant to the terms of the immunity agreement. If the prosecutors made a lousy deal, intentionally or just from incompetence, I still see the judge siding with the defendant most of the time.
Most immunity deals demand strict compliance from the defendant but I am not sure if the prior administration gave what amounted to practically a pardon. If a judge was convinced there was something nefarious going on that amounted to trying to skirt the law in collusion with the defendant, he/she might find prosecutorial misconduct but that’s a heavy lift.
As to Trump just reshuffling the deck and no longer honoring agreements, that wouldn’t happen. He’s head of the executive branch but a criminal matter would most likely come under judicial branch or possibly the state government. The trial judge and appellate courts run those shows.