The canon changes in The Winchesters spinoff may be explained by a letter from John Winchester's father, which leads him to discover the Men of Letters and his future wife, Mary Campbell, who is a hunter. A mysterious messenger who gives John the letter could be a time-traveler or someone like Chuck, who could manipulate the timeline. The theory that Dean passes on the message from his grandfather is more convincing, as it ties into Supernatural's existing canon and addresses Dean's wish for his father to know the truth about his disappearance.
