Modelling and problem posing can be seen as activities that mutually support each other. The aim of this study was to investigate the problem posing processes that take place when a modelling problem is reformulated and how these processes target modelling competencies. Sixteen ninth- and tenth-graders were asked to reformulate a modelling problem, and their solution processes were analysed. The findings showed that students extensively engaged in problem variation. They set up and varied situational and numerical assumptions and discussed how these assumptions influenced the authenticity and difficulty of the problem. The findings contribute to a better understanding of the processes involved in reformulating modelling problems and provide a basis for developing teaching approaches that target specific modelling competencies, such as making assumptions through problem variation.