Teacher identity has increasingly become a major educational research area. Nevertheless, high school in-service English teachers' identity changes, as facilitated by their further studies in the pursuit of professional development are less explored. This study begins to fill this gap by examining their identities as to how they strike a balance as a teacher and a student concurrently. Based on interviews with 11 teachers at a high school in Taiwan, both positive and negative changes were reported by the teachers, despite wide-ranging foci. Their productive and additive identity changes were primarily related to their sense of competence and fulfillment in terms of enhancing professional knowledge, increasing research capacities, and broadening horizons, whereas subtractive and split changes were mainly related to disciplinary differences. It can be concluded that teachers' identities are constructed and reconstructed as they seek to negotiate between their own beliefs and the natures of different disciplines. Implications of this study are discussed.