Due to design and manufacturing delays in the LM, Apollo 8 and Apollo 9 swapped prime and backup crews, and Armstrong's crew became the backup for Apollo 8.
dˈuː tə dɪzˈaɪn ænd mˌænjuːfˈæktʃɚɹɪŋ dᵻlˈeɪz ɪnðɪ ˌɛlˈɛm, ɐpˈɑːloʊ ˈeɪt ænd ɐpˈɑːloʊ nˈaɪn swˈɑːpt pɹˈaɪm ænd bˈækʌp kɹˈuːz, ænd ˈɑːɹmstɹɔŋz kɹˈuː bɪkˌeɪm ðə bˈækʌp fɔːɹ ɐpˈɑːloʊ ˈeɪt.