Late Airco DH.2s usually get a 110 hp LeRhone (as opposed to the original 100 hp Gnome rotary on earlier models). Also, in most cases, the gravity fuel tank on later models is located slightly left of center on the top wing (sometimes also left of center on the bottom of the top wing); as far as I know, earlier DH.2s have the tank right above the center section of the top wing. Other possible changes -- Lewis gun magazine storage boxes are located on the inside, rather than on the outside, of the fuselage on later model DH.2s. And last but not least -- worth considering is the type of prop that later model DH.2s get (typically the four-bladed prop after Jan/Feb of 1916; with the two-bladed prop being more prevalent earlier on).
Cheers & happy flying in the "Spinning Incinerator,"