Neil Armstrong December 23, 1968, as Apollo 8 orbited the Moon. In a meeting that was not made public until the publication of Armstrong's biography in 2005, Slayton told him that although the planned crew was Commander Armstrong, Lunar Module Pilot Buzz Aldrin, and Command Module Pilot Michael Collins, he was offering Armstrong the chance to replace Aldrin with Jim Lovell. After thinking it over for a day, Armstrong told Slayton he would stick with Aldrin, as he had no difficulty working with him and thought Lovell deserved his own command. Replacing Aldrin with Lovell would have made Lovell the lunar module
Apollo 8 CMP, and Buzz Aldrin as LMP for the third crewed Apollo flight was officially announced at the same time as the prime crew. When Lovell was reassigned to the prime crew, Aldrin was moved to CMP, and Fred Haise was brought in as backup LMP. Armstrong would later command Apollo 11, with Aldrin as LMP and Collins as CMP. Haise served on the backup crew of Apollo 11 as LMP and flew on Apollo 13 as LMP. During Projects Mercury and Gemini, each mission had a prime and a backup crew. For Apollo, a third crew of astronauts was added,