Assessment innovations require explicitly challenging teachers’ assessment conceptions. In changing towards outcome-based curricula, aligning formative to new summative assessments is a challenge. This study, conducted in Dutch Agricultural Vocational Education and its new outcome-based summative assessments, aims to provide concrete insights into what teacher conceptions hinder the development of aligned formative assessment practices. In a structured collaborative action research, practitioners, researchers and consultants analyse teachers’ formative practices, identify underlying current conceptions and their misalignment to the new summative assessment system. An iterative process resulted in an overview of current conceptions versus required conceptions, showing required conceptual changes that are prerequisite for teachers to change towards formative assessment practices that are aligned to the new outcome-based summative assessment system.