Background: Learning assistants (LAs) are undergraduate students who serve as instructional assistants in STEM classrooms. In addition to engaging in active practice, they take a pedagogy seminar and regularly meet with a content instructor. While aspects of LAs' pedagogical beliefs and actions have been investigated, there remains a gap in understanding how LAs make sense of their new instructional roles and how they negotiate between their experiences as students and their responsibilities as instructors. This study uses a sequential, exploratory mixed-methods approach, which includes constant comparative open-coding, thematic analysis, and epistemic network analysis, to analyze 178 reflections written by 89 LAs across five terms at two institutions. Here, we identify each LA's expressed goals and intended actions at the start and the end of their first term as an LA. Using a community of practice framework, we seek to explicate the shifts in these LAs' values as they become more central members of the LA community. Results: LAs' expressed roles shift from being cognitive coaches, where they focus on student thinking, sense-making, and understanding of disciplinary concepts, to being social architects, where their focus shifts to attending to the aspects of the environment that can support productive interactions for learning. A social architect prioritizes goals related to mutual trust, respect, & approachability, understanding and learning about students, and creating a sense of belonging. Similarly, their intended actions emphasize compassion, understanding, and facilitating group discussion. While all LAs studied exhibited this shift, it manifested in different ways and to different extents, as illustrated in detail by four selected cases. These cases illustrate how the shifts coupled to a change in language around teaching, becoming more specific and contextual. Conclusions: LAs express a shift in their valued practices over their first term as LAs related to their instructional role. The goal of student-centered instructional practice is often framed as becoming a better cognitive coach; however, this orientation does not foreground ideas around teaching practice that aim to foster engagement, belonging, and student agency. Implications for both the LA model and, more generally, for postsecondary STEM instructors are discussed.