Changing leaders mid term isn't unusual:
1902 AJ Balfour (Con)
1908 AA Asquith (Lib) - Won in 1910
1916 Lloyd George (Lib) - Won in 1918
1923 Stanley Baldwin (Con) - Won in 1924 and 1935
1937 Neville Chamberlain (Con)
1940 Winston Churchill (Con) - Won in 1951
1963 Sir Alec Douglas Home (Con)...