A GDO conversion assembly for transitioning between a ceiling mounted and a jackshaft configuration includes a motorhead having a motor output shaft extending from a motorhead casing that includes a mounting interface disposed proximate to the motor output shaft, a track assembly including an input interface shaped to mate with the mounting interface to operably couple the motorhead to a sectional garage door, and a transmission assembly including an input coupler shaped to mate with the mounting interface to operably couple the motorhead to the sectional garage door. The transmission assembly includes an output coupler to operably couple the transmission assembly to a drive tube rotatable to alternately open and close the sectional door. The output coupler includes an output pinion. The input coupler includes an input pinion operably coupled to the output pinion via a flexible coupling member. The transmission assembly includes a transmission housing and a coupling bracket.