We harnessed a large-scale randomized controlled trial to investigate if and how experienced Advanced Placement (AP) Environmental Science and U.S. Government treatment teachers would shift their practice towards the Knowledge in Action project-based learning (PBL) approach. In any instructional context, shifting to project-based learning can be difficult for teachers and students. The AP curriculum framework breadth and examination focus may exacerbate the challenge. Under conditions of optimal support and compared to randomly-assigned, business-as-usual control teachers, treatment teachers shifted towards greater emphasis on deeper learning objectives, more student-centered pedagogy, more authenticity, and less lecture and explicit examination-preparation. Though the shift was challenging, students reported feeling prepared for the relevant AP examination, and over ninety percent of teachers recommended the approach.