Larry Gomez on the change announced with "The Origin of the Work of Art".
In the Origin essay, we see Heidegger's shift in thinking from Being and Time because he no longer locates the question of truth in the being of Dasein, but now in Being itself. Truth is no longer a structure of Dasein but the way Being unfolds in a given epoch. As such, it is subject to the flow of time, but in its unfolding results in an event, something that happens at a certain place in a certain time - "the site of the moment" as he calls it in Contributions. For Heidegger, this event of truth is simultaneously the grounding of that event in history. And art is one essential way in which that the event of truth arises and thus configures history. Heidegger says, "The origin of the work of art... is art. This is so because art is in its essence an origin: a distinctive way in which truth comes into being, that is, becomes historical." [P. 202] Thus, for Heidegger, art is the pathway through which truth comes into being. But the coming into being of truth is historical because it changes throughout time. Heidegger comes to the same foundation when lecturing on Parmenides only he reverses the order of truth and history. There he says "[t]he 'meaning' of history, however, is the essence of truth, in which at any time the truth of a human epoch is founded." [P. 56] The rest of that lecture reflects on this meaning of truth which was found in Parmenides' fragments at the dawn of western philosophy.
This historical dimension of truth is a shift from Heidegger's philosophy in Being and Time. With the Origin essay, the truth of becomes something historical. In Being and Time, Heidegger recognizes the historical as something in the past which no longer functions the same way in the present. The antiquities in the museums are still present-at-hand in the museum because visitors can see them, but they no longer function as they once did because the world they functioned in no longer exists.
In Being and Time what grounds history and also the world is Dasein. Dasein is what is primarily historical, while artifacts, ruins and landmarks are only secondarily historical. This grounding of the historical in Dasein is what changes in Heidegger after Being and Time when he drops talk of Dasein and focuses exclusively on Being, language and truth. We see this shift Origin where is no mention of Dasein. Instead, it is truth and Being which make up the historical.