This study examines teachers' discussions in a professional development setting to understand the ways in which learning a mathematics learning trajectory may change aspects of their discourse about students as learners. Using mixed methods, we bring together two theoretical frames that use a Vygotskian perspective on learning to analyze professional discussions among 22 elementary-grade teachers participating in a yearlong, 60-hour mathematics professional development program. Results indicate that over time, some discursive patterns for explaining students' academic performance changed to incorporate the trajectory, while others remained unaffected. Whereas this change transformed one of the patterns in a way that led to new explanations for student performance, another pattern changed only slightly and was still used to express the same explanations for performance.