Richard Capobianco has added an excellent paper to his web site, "The Fate of Being in Heidegger's Four Seminars 1966-1973". It deals with die Sache selbst [the central issue] for Heidegger, one of Thomas Sheehan's central theses in the new paradigm, by examining Heidegger's many meanings of being; as the being-ness of beings, the being of beings, being as being, and so on. Read the whole thing.

This bit dicusses which "being" pertains to Ereignis.

The discussion continues with Heidegger further clarifying the meaning of das Ereignis, and the same considerations we have addressed come into play. To think das Ereignis is to “step-back” from thinking about “the various epochs of the history of being (Geschichte des Seins).” Here, again, he means more precisely that the “history of being” is the history of the epochal renderings of the being-ness (Seiendheit) of beings because he explicitly adds that these “are the epochs of the various ways in which presence (Anwesenheit) dispenses itself to Western human beings” (61:367) from the Greek period to the contemporary period (and continuing forward). As he sees it, then, thinking must “go beyond” this history in order to think “Being as Being,” which even the Greeks could not quite achieve. Thinking that goes beyond metaphysics thinks das Ereignis as the temporal unfolding—which also withdraws itself from view—of beings (including human beings) in their being-ness as this comes to pass in all the different epochs. Thus, thinking das Ereignis is thinking Being as Being. And by virtue of this thinking, we become aware that “the history of being[ness] has not so much reached its end, as that it now appears as the history of being[ness].” One important lesson of this seminar discussion is that we realize that Sheehan, Maly, and some other commentators have claimed too much for the notion of das Ereignis. Thinking das Ereignis is not more fundamental than thinking Being as Being (Being itself); rather, fundamental thinking thinks Being as Being (Being itself) as das Ereignis.

So, when Heidegger refers to events of historical changes in metaphysics, he is referring to changes in the way the being-ness of beings is understood. But there is always an Ereignis, an opening of beyng, in thinking.  ¶ 3:05 PM