In recent years, Asian countries including South Korea have been trying to redefine the role of English in response to globalisation. The impact of this on English language teaching and on Korean society more generally has been well documented; however, the impact of this change on individual teachers and their teaching calls for further attention. This article investigates the impact of an in-service teacher certification scheme which promotes English as the medium of instruction (MOI) for teaching English in the South Korean context, and examines the factors which shaped this impact. Drawing on interviews with stakeholders and insights from a larger case study of which these interviews form a part, this article provides important insights into the implementation and impact of language planning and policy (LPP). It examines the ways in which particular elements of the design of LPP, their implementation and the context interact to affect teachers and their adoption of English as the MOI. It also draws attention to unplanned impacts of LPP including significant shifts in teachers' identity. It concludes with wider implications for future LPP which aims to change the relative importance and roles of a foreign language.