Greg Sargent @ WaPo summing up why this is headline stuff:  Gordon Sondland just gave us this scandal’s smoking quid pro quo. It's devastating the "anti-Trump deep state conspiracy" excuse:

[....] To sum up: American officials, acting at the instructions of Giuliani, who was in turn acting at the instructions of Trump, held up military aid desperately sought by a vulnerable ally under Russian military attack to coerce Zelensky into carrying this whole scheme to fruition.

This testimony is especially damaging coming from Sondland. Nearly all the others who have given damaging testimony about Ukraine are career foreign policy and national security officials. Trump has derided them as “Never Trumpers.” Even if that allegation is a scurrilous one, he couldn’t say it about Sondland [....]

Sargent also implies it was Amb. Taylor's testimony that was the catalyst in Sondland revising his testimony:

[....] But then Ambassador William Taylor, who texted contemporaneously about his alarm over exactly that quid pro quo, confirmed it in deeply damning testimony. And White House national security adviser Tim Morrison, who had been discussing these matters with Taylor, confirmed Taylor’s account.

Which apparently jogged Sondland’s memory. In a new statement appended to the transcript of his testimony, Sondland clears it up once and for all:

Also, I now do recall a conversation on September 1, 2019, in Warsaw with [Zelensky aide Andriy] Yermak. This brief pull-aside conversation followed the larger meeting involving Vice President Pence and President Zelensky, in which President Zelensky had raised the issue of the suspension of U.S. aid to Ukraine directly with Vice President Pence. After that large meeting, I now recall speaking individually with Mr. Yermak, where I said that resumption of U.S. aid would likely not occur until Ukraine provided the public anti-corruption statement that we had been discussing for many weeks.

[....]