I have not read about this subject for a while but I seem to remember that copilots would be eligable for upgrading after a certain number of missions. So I guess that the replacements were partially from within the squadron and also from the new crews that were fresh from training. Just my two cents.

By the way, I have always enjoyed John Comer’s ‘Combat Crew’ as a great read and a very honest insight into the lives of these crews.