12-1-41 (that's January 12,) as modification 269 on the Spitfire; probably about the same time (modification 190) on the Hurricane.
A Spitfire modification leaflet was issued, enabling units to do the work themselves (15 man-hours,) but not until March 1941.
The change was made when trials against a Me109 found that the pilot could withstand tighter turns, without blacking out, which was put down to his straight-out leg position.
Bader had a similar advantage, since he had no legs for the blood to drain into.