Reforms such as the Next Generation Science Standards (NGSS) call for substantial change in teacher practice such as instruction that is more phenomenon-based, three-dimensional, supportive of student epistemic agency, and coherent from the student perspective. Such change requires ongoing support in the form of aligned curricular materials and professional development (PD), but little work has been done to understand how teachers respond to PD over time in the NGSS era. Therefore, this study investigated how teachers' beliefs about science instruction and self-efficacy in implementing a reform science curriculum changed over time. Using survey data from the field test of the OpenSciEd middle school curriculum implementation, we tracked changes in teaching beliefs and self-efficacy beliefs in 322 teachers over 2 years, which included four rounds of PD workshops and implementation of four new curricular units. Teachers' teaching beliefs and self-efficacy beliefs changed initially and leveled out over time, but self-efficacy took longer to change than teaching beliefs. This suggests that teachers may require more time, iteration, or enactment to increase their confidence in implementing new instruction as compared to their beliefs in the value of the new approach. Using hierarchical linear modeling, we identified teacher characteristics associated with differences in those changes. Teachers who had more experience and found the PD more valuable were less likely to hold traditional teaching beliefs and more likely to have higher self-efficacy. Of these variables, teachers' value of the PD is particularly interesting because it can be designed for. Based on these results, we argue that PD should be designed to build trust and to provide a rationale for PD activities for participants to help understand the value of the experiences.