Michele Heidenberger

At least two of the four flight attendants who will be on American Airlines Flight 77—the third plane to be hijacked—on 9/11 are not originally scheduled to be on this plane, and are only assigned to it shortly before—or early in the morning of—September 11:
Michele Heidenberger regularly flies from Washington’s Reagan National Airport to Dallas. But she will tell a colleague she is working Flight 77 on 9/11 as she is building up vacation time so she can go to Italy in October. [Murphy, 2006, pp. 42-43]
Renee May is assigned to Flight 77 early in the morning of 9/11. Before she is offered the flight, American Airlines asks another attendant, Lena Brown, to take it, but Brown says she is unable to get to the airport in time. [Atlanta Journal-Constitution, 8/9/2002; Daily Reflector, 9/11/2002]
One of the pilots—either Charles Burlingame or David Charlebois—is also not originally scheduled to be on Flight 77 on September 11, and only accepts the flight shortly before then (see Shortly Before September 11, 2001). [New York Times, 9/13/2001] Many of the flight attendants who will be on the other three hijacked planes are also only assigned to those flights shortly before 9/11 (see Shortly Before September 11, 2001-Early Morning September 11, 2001, Shortly Before September 11, 2001, and Shortly Before September 11, 2001).