WaPo archive | Obama Gives Political Ambassadors Their Pink Slips, 3 Dec 2008
"The incoming Obama administration has notified all politically-appointed ambassadors that they must vacate their posts as of Jan. 20, the day President-elect Barack Obama takes the oath of office, a State Department official said."

reference
US case law, The Removal Power (1926), "with the exception of federal judges"
USOGE, Political Appointees, "any employee who is appointed by the President, the Vice President, or agency head"
National Archives, The Nomination Papers of the United States Senate, 1789-1946