Improving inter‐agency working across organizations is an important goal across public and private sectors. The UK Emergency Services have spent a decade implementing organizational change to improve interoperability between the Police, Fire and Ambulance Services. JESIP—the group tasked with realising this change—have faced criticism. We evaluated JESIP's efforts by interviewing expert commanders, finding participants supported the principle of change, but issues impeded its implementation. We developed the Principle‐Implementation Change Framework for Interoperability (PICI) to describe the gap between change principles and change implementation, identifying the macro‐systemic, meso‐organizational and micro‐psychological processes between them. Key obstacles to implementation included macro‐level funding issues, incompatible meso‐level organizational structures and strained micro‐level peer‐to‐peer relationships. Participants also reflected on the facilitators of change. At the meso‐organizational level, JESIP was perceived to have improved inter‐team communication and flexibility. At the micro‐psychological level participants described enhanced trust, shared identities and the emergence of a new type of interoperability leader. This study highlights the importance of gaining support for the principle of interoperability while addressing implementation challenges posed by the inherent social complexities involved in this change. Change efforts must be monitored over time, considering the macro, meso and micro‐level processes that influence the principle‐implementation gap.