What I'm really looking for is scenarios that wouldn't lead to a halt in the program like the 204 fire, but merely a crew exchange
Now i get it !
back to topic what you define with "loss" in crude that also disease ?
in this case we have Apollo 7 and 13
in Apollo 7, Schirra developed a severe head cold during mission and infected his crew mates in capsule
hat that happen during quarantine before launch they had be grounded and Backup crew take over
in Apollo 13 CM pilot Ken Mattingly, was exposure to German measles. so the flight surgeon grounded him and John Swigert took his place.
The prime crew could get grounded in fear they all could get sick in space (see Apollo 7) and Back up crew take over
Other issue
Training on ocean goes wrong or
crew died in Aircraft crash fling, between NASA center, car crash
swindle like Apollo 15
Here the Crew smuggle unauthorized commemorative postal covers in there space suit, next several wrist watches to test them in Space.
also the "Fallen Astronaut" statuette, all that were not permitted by NASA
had the NASA discovered this scam prior to Launch, the Crew had be grounded and lay off