English as a lingua franca (ELF) represents a substantial paradigm shift in ELT because it challenges the traditional norm of standard English. Despite this changing trend, there is a gap in research on incorporating ELF into in-service teachers' professional development (PD). Given the backdrop that (trans)disciplinary courses are expected to be taught in English in compulsory K-12 education due to the Bilingual 2030 Policy in Taiwan, this paper proposes an ELF-informed PD model and specifically examines English teachers' perspectives before, during, and after their participation in the first nationwide bilingual in-service teachers' PD programme. We observe a growing recognition of ELF concepts among teachers, which is evident in their understanding of the pedagogic challenges and implicit tensions in ELF practice, their validating senses of being ELF teachers, and their critical reflection on ELF-informed pedagogy. The enhanced understanding highlights an alternative approach to transforming the 'E' in EFL.