In this article, we investigate the change forces that act on administrators, subject department chairpersons and teachers as they seek to implement a change in a Canadian secondary school. Using a case study methodology, our analysis of the data uses Sergiovanni's (1998) six change forces: bureaucratic, personal, market, professional, cultural, and democratic forces. Our interpretation supports the importance of the principal and administrators, working together with teachers, in implementing change. The analysis points to the chairperson of subject departments having a crucial, but often overlooked, role in the implementation of change. Three key co-requisites that allow chairpersons to play this critical role are: the existence of a school-level democratic commitment to the common good that guides the work of professional learning; the location of professional learning within departments to operationalise the common good; and, the capacity of the chairperson to fulfil their role as an instructional leader in the fullest sense of the term.