This paper examines the unintended consequences of state language policy and planning (LPP) that adopt subtractive approaches on teachers' subsequent receptivity to policy fine-tuning. A comparative approach is adopted in this statistical study of two strategic contexts, where the influence of the world's two leading languages--English and Mandarin--manifests in the home language conversion patterns of ethnic Chinese teachers of Hong Kong and Singapore. The interplay among state, education, and family linguistic domains provides the framework to understand how teachers exercise agency underpinned by their sociolinguistic background (childhood home language--CHL) and home language conversion preference (home language as adult--HL). The results show that teacher CHL-HL conversion preferences underlie their response to state LPP initiatives and influence LPP outcomes in the education domain. The results are theorized in terms of the prevailing values in Hong Kong and Singapore that shape teacher agency, the unintended outcomes of subtractive LPP in education, and the probable outcomes on the linguistic vitality of local and dominant languages in Hong Kong, with the interplay between future subtractive LPP and teacher agency.