That's definitely a possibility. What then? How does that play out?
Insubordination gives Trump cover for firing Rosenstein.
The President has Constitutional authority to declassify anything he chooses. If Sessions/Rosenstein disobey a direct order from the President regarding such a request, I would suspect a conversation something like this:
"Mr. Sessions you have 2 choices, fire Rosenstein and execute my request or I will fire you and Rosenstein and replace you with someone who will respect my office and the Constitution. You decide NOW what you will do!
Contempt of Congress.