Assuming the declass statement is true as written, Barr has control over how and when things come out. (Trump may have already given him some “guidance” on the that).
How will it happen, fast, slow, medium?
And in what order?
And how much of it will be completely new to everyone?
I suspect he has given some 'guidance'. Also worth noting that the President is taking this action at the 'request and recommendation of the Attorney General. AG Barr is requesting the declas.