There were various times in 1940 when Churchill might have been replaced, but not by Attlee; the Conservatives had more than double the seats in the Commons that Labour had. Churchill's replacement would have to either come from within the Conservative party or be someone who was trusted by them ahead of anyone in their own ranks, as Lloyd-George was after 1918. Attlee wasn't trusted by Tory members in that way; he owed his presence in the cabinet to Churchill alone.