section 1264 basically says that he has to tell Congress when he makes a change to policy within 30 days of doing so.
But the letter he sent to them is just a cover letter, he didn't make public what changes he has or is going to make.
So we can only speculate - go to the latest Report to Congress….
https://www.state.gov/wp-content/uploads/2019/10/Report-to-Congress-on-legal-and-policy-frameworks-guiding-use-of-military-force-.pdf
Reading through this, it talks about ongoing military action in Syria, Lybia, Afghanstan, Iraq, Niger, Somalia and Yemen.
So either Trump's making a critical change in the armed actions in one or more of those places, for which he has to tell Congress, or there's one other thing in the Report, the last item.
It talks about Capture and Detention of Individuals, and about Gitmo.
So maybe some big change there, I don't know. Will just have to wait and see when he makes it public.