© Andriy Dubchak/RadioSvobodsMarie Yovanovitch, former US ambassador to Ukraine
The State Department issued a December 2008 notice to Bush-appointed ambassadors to submit their resignations, effective when President Barack Obama took office in 2009, according to the Washington Post.
A State Department official told Agence France Presse at the time that demanding all previously appointed ambassadors resign was routine.
"It's a normal procedure for ambassadors, career and non-career, to submit their resignations. And what happens is that all of them do," the official said at the time.
Typically, some career ambassadors are later allowed to stay in place on a case by case basis until they are replaced.
Trump also issued a notice asking all Obama-appointed ambassadors to submit their resignations when he prepared to take office, which created controversy at the time, as it appeared that he would not allow extensions.
Yovanovitch was first appointed by Obama in 2016 and confirmed by the Senate. She was asked by the Trump-run State Department to stay on as an ambassador to Ukraine through 2020, but she was fired in May 2019.
Unlike Yovanovitch (appointed by one administration, replaced during another administration) others have been removed by the same administration that appointed them. Former Amb. John Evans was removed from his post for refusing to deny the veracity of the Armenian Genocide. The way Wikipedia begins the reportage,
"John Marshall Evans (born 1948) served as United States ambassador to Armenia. He was confirmed to this position by the U.S. Senate on June 25, 2004. Evans began his service on August 8, 2004, but, as confirmed by President George W. Bush on May 24, 2006, was terminated for undisclosed reasons."