This article examines the impact of a professional development program aimed at encouraging high school teachers to include more modern physics in their enacted classroom curricula through improved content and pedagogical knowledge. The program was implemented with 17 high school physics teachers (three were the focal point of this study) in Turkey. A multi-case study using mixed-methods analysis followed by a cross-case analysis revealed conceptual and pedagogical gains. Teachers' higher scores on an achievement test indicated their development of increased subject-matter knowledge. Further, classroom observation revealed a shift from lecturing to dialogic practices centered on student sense-making such as discussion, questioning, and interactive use of technology (simulations, videos). In alignment with the professional development, teachers devoted more instructional time to modern physics, increased the conceptual scope of their lessons (more concepts were taught), and adopted instructional approaches that were more dialogic. It is argued that the combined use of legal mandate and teacher development can help address the problematic status of Einsteinian physics as an absent curriculum at the high school level.