This article addresses the issue of policy change and its consequences on the organizational field and institutional context, looking at the role of an instrument transfer process. We adopt a political sociology approach to policy instruments to account for the impact that the introduction of a securitarian instrument, the mayoral order, had on the implementation process of a social policy. Through a genealogy of the instrument we identify a constitutive, normative and cognitive dimension. Our aim is to explain the interaction between these institutional dimensions, the re-definition of the organizational field of the policy, and the dynamics of metropolitan politics. Following a synthesis of the policy process and the institutional arrangements where the policy is implemented, we show how the instrument transfer triggered a division within the organizational field, and the rise of institutional tensions amongst local authorities. In the conclusion, we identify the institutional dimensions of the instruments and the policy implementation scale as elements that are to interact with public-private relationships and institutional arrangements, and that have an impact on implementation process and policy outcome.