It's interesting that Aldrin was backup CMP on Apollo 8 but became prime LMP on Apollo 11. I wonder why. All the astronauts were given an area of Apollo systems in which to specialize, and Collins' was pressure suits. That would seem to make him a candidate for LMP. On the other hand, Aldrin had done an exceptional job in preparing for his EVA on Gemini XII, and as Dr. Rendezvous he was an obvious choice for a LM crew. Does anyone know for sure how Aldrin moved from the CMP role to LMP? And does his earlier position as backup Apollo 8 CMP explain why he rode in the center seat during the Apollo 11 launch?
The way Mike Collins tells it, Deke Slayton moved in mysterious ways. The shuffling of crew assignments got a little complicated, for sure.
Collins was originally assigned as backup LMP (with CDR Frank Borman and CMP Tom Stafford) for a flight that never happened - it would have been the next flight after Grissom's and would essentially be a repeat of the first flight.
NASA management decided to cancel that flight because they saw no need for a duplicate flight; remember the time pressure they were under. So the prime and backup crews were broken up and reassigned. Collins ended up "promoted" to CMP on Apollo 8 with CDR Borman and LMP Bill Anders. He was made CMP because Anders had never been up and Deke had an ironclad rule at that time that CMPs must have flown before.
So, from 1966 on, Collins became a specialist in CM systems - as he put it, "all the things I was least interested in doing". Then in July, 1968 - 5 months before AS-8 was to fly - Mike had to have neck surgery and was, of course, dropped from the crew.
Now. The backup crew for Apollo 8 was Armstrong (CDR), Lovell (CMP), and Aldrin (LMP). When Collins had to drop out, Lovell moved up to the prime crew CMP spot. Deke made the decision to move Aldrin to CMP and bring Fred Haise in as backup LMP. Buzz, presumably, promptly began cramming like a madman for the CMP role.
So, when it came time to assign a crew for AS-11 (Collins was back on full flight status by then), Slayton assigned Armstrong as CDR, put Buzz back in the LMP slot (which he had originally trained for), and Collins as CMP.
And, yes, Buzz rode the center seat during the 11 launch because he was more recently trained up on that slot and Collins knew the CM so well he could pretty much handle any position (necessary, since he might have to fly home alone).
SOURCES: Carrying The Fire, plus Wiki for some dates and details.