0) To a lesser extent, the death of Ted Freeman, which freed up a
slot. However, Freeman's death occurred early enough in the program
that, IIRC, he hadn't been pencilled in on any rotation. Note that
most, if not all of the Astronauts who were part of the program at
that time were convinced that had Freeman lived, he would have been in
line for Apollo and more likely than not one of the Lunar missions.

In Lost Moon/Apollo 13, Lovell refers to Ted as someone considered a strong
contender for the remaining Gemini flights that would have members of Group 3
aboard. Though his assignment to support Apollo might suggest otherwise (a
couple Apollo support guys did fly on Gemini or work backup however).

1) The deaths of Eliot See & Charlie Bassett, which bumped Neil up
from backup to prime crew for G8, and shuffled around the rest of the
Gemini rotation, and put Buzz into a prime crew spot.

I've played around with the crew assignments now and then and as near as I can
figure this factor was something that pushed Tom Stafford from landing a later
Apollo slot...had he commanded Gemini 12 he would have moved over to Apollo
later and commanded a later mission...that later mission could have been Apollo
11 or 12.

2) The A1 fire, which delayed the program long enough for mission
reshuffling and cancelling - ie, AS-278 - to shuffle the rotation
again with three less Astronauts in line.

That pretty much led to the basic creation of the Apollo 11 crew. All the
backup crews moved up post-fire and to fill the void Deke assembled a crew of
Armstrong, Lovell and Aldrin.

3) The delay in the LM, which resulted in A8 & A9 being switched, A8
being redefined totally, and the resulting rotation that up to that
point pretty much had Pete Conrad and CC Williams in line for the
first landing attempt.

That swap actually occured after C.C. died and was replaced by Al Bean.

-A.L.