Understanding place value is a critical and foundational competency for elementary mathematics. Classroom teachers who endeavor to promote place-value development adopt a variety of established practices to varying degrees of effectiveness. In parallel, researchers have validated models of how young children acquire place-value understanding. Faced with competing demands, teachers may resist abrupt, disruptive, or time-intensive changes to existing practices. In this study, we explore ways of introducing changes in pedagogical supports that minimally disrupt existing classroom routines that nonetheless offer potential gains in children's understanding of base-ten number concepts. Specifically, we examine the impact of modifying how teachers demonstrate and discuss representations of the ten-for-one structure during the familiar "Days in School" counting routine on young children's place-value understanding. We report on a 2-year longitudinal study of a cohort of students beginning in grade one, conducted in nine classrooms across a control school and a treatment school. We describe the modifications in the counting routine and report the results of children's development of base-ten number concepts. The percentage of children who identified base-ten number concepts was three times higher for the children in the treatment group than for the children in the control group. Moreover, nearly twice as many children in the treatment group as compared with the control group advanced beyond a concatenated single-digit conception by the end of second grade when counting two-digit quantities. We conclude with a discussion of the implications of our findings for early elementary mathematics education more generally.