Currently, the inner dynamics of teacher identity transformations remain a "black box." Conceptualizing preservice teacher identity as a complex dynamic system, and the notion of "being someone who teaches" in dialogical terms as involving shifts between different teacher voices, the study investigates the dynamical processes at play when transitions between identities occur. Using a single-case design, and drawing on intra- and inter-personal data collected across three timescales, the identity transformations of a preservice teacher during a practicum are examined as a process in motion. The study offers a systemic account of the participant's teacher identity experiences, analyses revealing oscillations between two identity positions and a pattern of multi-stability. It is suggested that complexity approaches can be valuably used in mentoring processes to help students make sense of identity transformations and the conditions under which they occur. In the longer term, support of this kind can have a positive impact on teacher retention.