As teaching is a cultural activity embedded in a unique context, professional development opportunities for teachers may be best placed amid the relevance of everyday classroom practice, and supported by an innovative curriculum. This progress paper reports on data collected as part of doctoral research studying the changes in mathematical knowledge and beliefs of three year 5/6 teachers as they implemented a four-week, innovative curriculum unit. Early analysis of the case of Year 5 teacher Mark pointed to reflection on the pivotal influence of teachers' stated and inferred beliefs about mathematics and mathematics teaching on classroom practice, and the vagaries of didactic contracts in a change environment.