The purpose of this study is to understand the nature of pre-instructional knowledge transferred by students into problem situations and the change process on students' knowledge system during classroom discussions. This study was framed by two interrelated theoretical frameworks on knowledge structures, phenomenological primitives and coordination classes. The data were collected through problem solving sessions on turning effect of forces (torques or moment) from ten participants who were seeking a degree to become physics teachers. The analysis of data showed that, in this particular context, students' pre-instructional ideas can be characterized according to phenomenological primitives. The theoretical constructs of the coordination classes generated meaningful results to understand students' particular difficulties in transferring the moment concept across different contexts and the change process on students' knowledge system. The major stimulator of the change process emerged as the students' becoming aware of the epistemological nature of their knowledge structures and searching the causal mechanisms behind physical phenomena.