There doesn't seem to be an exact indication of when the move was made, but it is treated as a new escalation. Using outside experts to review Trump's corporate records can be a prelude to using these experts as witnesses for upcoming legal action. The timing certainly seems noticeable. Trump will be stepping down in just three weeks. At that point he will lose the president's alleged "immunity" from criminal charges blocking any investigation. He will undoubtedly be indictable by the afternoon of January 20th.
But there are three weeks between now and then, and Trump's criminal of self-serving employee pardons will no doubt continue. He has already pardoned anyone who lied to federal investigators about his campaign's efforts to take advantage of and benefit from the Russian government's espionage efforts aimed at sabotaging his opponent in his 2016 presidential bid. It is believed that the next pardons will broadly be aimed at immunizing himself and his children against the financial crimes exposed in past press reports, but it will only serve to block federal, not state, charges. This is New York investigating its business, not the always compliant Department of Justice.
So we can read the tea leaves here, but … hmm. It doesn't tell us much. It is possible that District Attorney Cyrus Vance Jr.'s office hired new outside help because something special was found in the Trump Organization's finances. It's just as possible that they were hired because the office's in-house experts simply lacked the ability to follow and guide every number. It is possible that this new escalation has been leaked on purpose to signal that the investigation is in full swing and prepared for Trump's impending loss of presidential privilege. It is very likely that it shouldn't be phased out as the currently smoking and unstable Trump has three weeks to ponder whether there are additional powers of the president to use in retaliation.
Granted, Trump is currently busy trying to overthrow the United States government, but investigations that could one day jail him still seem to be getting his immediate attention.
All we know is that in New York, the investigation into whether Donald Trump made the fortune he still has by being a lifelong crook is not only going on but is expanding. That will upset him again. This probably still ends up with him fleeing the country claiming he's just too good for us and we're too mean to him. It will hurt him to leave Mar-a-Lago, but he left his New York tower on far minor insults. And while he wants to start a new presidential campaign, there is nothing to say that his esteemed base will not continue donating to this cause even after Dear Leader strolls to a sympathetic kleptocracy.