In this mixed methods study, we use positioning theory to analyze teachers' professional discussions about students' mathematical work over the course of 60 hours of professional development focusing on a learning trajectory. After identifying a category of speech actions that suggested student learning is limited by their age or grade level, a quantitative investigation revealed that teachers' discussions grew to include elements of the learning trajectory over time. A subsequent qualitative analysis revealed changes in the structure of these discussions, where teachers came to recognize and use students' prior experiences in instruction, and student learning was more influenced by prior experiences than their age or grade level. [For the complete proceedings, see ED583989.]