There are a couple of different issues at play here. The first is, basically, is it an illegal act if the president violates on of his own executive orders? It's a somewhat metaphysical question, and absent an argument to the contrary I tend to think that the answer is "no" unless some statute is being violated as well. However, that's not to say that violating your own executive orders is an appropriate thing to do.
On the issue of document declassification I think the issues are fairly clear. There are well established processes for declassifying information which, in part, involve running the documents by people who supposedly should know whether revealing the information could harm national security or harm intelligence assets or whatever. So, whatever legal right the president has to declassify information at will is separate from the issue of whether any competent president would go about doing such a thing.
Whatever the legal issues, the president bypassed normal declassification procedures - put in place to ensure that revealing information does not threaten national security - in order to wage a political battle. Whether strictly legal or not, it's an act of a man who puts himself above the country. For shame.
