The transition to new assessments aligned to the Common Core State Standards was a significant turning point in the standards' implementation. Concerns about the transition led districts to suspend the use of value-added scores for evaluating teachers, but changes to other measures, such as classroom observations, were rare. Using data from the Washington, DC Public Schools, I evaluate the effect of the assessment transition on teachers' practice. I find substantial declines in instructional practice, ranging from 13% to 20% of a standard deviation, for teachers in tested grades and subjects when the new exam was introduced. These results suggest that policymakers should consider the ramifications of testing changes on a wider array of teaching outcomes than value-added scores alone.