Originally the idea was that in order to awaken the MS one had to kill their best friend.
Later on it was clarified by Tobirama that Uchiha members possessed some kind of special gland in the brain that produced a special type of chakra when under intense emotional stress and trauma, which is basically what happened when they killed their closest friend or witnessed their death. Obito is a good example of that as he personally witnessed Rin die at the hand of Kakashi.
Nothing was retconned, just expanded upon.