If the plea deal is officially confirmed by a judge, then yes, Julian will be safe from being prosecuted again for the same charges if another administration were to change their mind about his fate in the future. This is due to the Double Jeopardy Clause, which is specified in the 5th Amendment:
Discussion
However, since the entire trial so far was a complete and utter joke, and they even contemplated outright murdering him before, I wouldn't count on this being an important detail. The only thing that really keeps him safe is as many people as possible knowing who he is, what he actually did, and what they did to him.
But will he be a witness?