To explore how teachers' beliefs and behaviour change throughout implementation processes, N = 47 teachers participated in a 6-month professional development (PD) programme, and then implemented a differentiated reading innovation over one school year (totalling 15 months). Teachers' beliefs (acceptability, feasibility, self-efficacy) and behaviour (fidelity, innovation use, adaptivity) were assessed after PD (T2), after 3 (T3), 6 (T4), and 9 months (T5) of implementation. Multilevel models displayed that acceptability and feasibility remained stable over time. Teachers' self-efficacy and adaptivity increased significantly, while innovation use and implementation fidelity decreased. Cluster analyses based on teachers' individual and school-level pre-conditions at T1 revealed one cluster with moderate and one cluster with optimal pre-conditions. The two clusters showed a parallel development with clear level differences, but teachers with moderate pre-conditions showed a steeper decline in fidelity after 9 months. Results emphasise that teacher change is a dynamic process that should be tailored to underlying conditions.