I'm not sure if the change is documented but it appears to have taken place during the large (17000 machines) contract C7287.
I have images which show that the large tank was in use at least up until C4348432 (WM20 31840) and that the smaller tank had appeared by C4349974 (WM20 33382).
So, somewhere around WM20 32000 - 33000 would seem to be a about the changeover. Henk probably has more detailed information from the factory records but my guess from the Ordnance delivery records is that the change must have occurred somewhere around early 1941.