Many tertiary educational institutes are adopting English as the medium of instruction (MOI) globally. In 2014, a publicly funded university in Hong Kong introduced a new language policy that marked a major shift in practice by switching the main MOI to English from Classical Written Chinese and spoken Cantonese, the predominant local language. The new policy caused some anxiety among staff and students as to how the change might be implemented effectively.This chapter analyzes this transitional process through a case study including surveys and interviews. After the introduction of the context in terms of the language-in-education policies and other changes, the case study findings are presented in terms of the perceptions of the reforms and challenges experienced by the staff and students. It then reports how these challenges were addressed through different support measures with some success, and the principles pinning down those initiatives. It also analyzes how the reform created impacts in unintended areas, such as potential mission drift. It concludes with a reiteration of the evolutionary nature of MOI and other educational reforms, and some suggestions on planning and managing those reforms to minimize the reform costs. Copyright © 2021 selection and editorial matter, Lily I-wen Su, Hintat Cheung, and Jessica R. W. Wu; individual chapters, the contributors.