Appropriation of technology is a process by which users complete the work of designers by making interactive systems functional within the frame of their situated activities. While existing theories and studies about appropriation are oriented toward the psychological or organizational dimension of this process, we propose a model to describe it through evolutions of digital artifacts and information structures. We also present a case study demonstrating how this model helps to identify particular user operations, and related digital transformations, as a part of the appropriation process. These findings open perspectives to bridge scattered theoretical approaches of appropriation around a low-level, artifact-oriented, and objective way of describing appropriation. Our model could also improve the way appropriation is taken into account in design, by bringing more focus on technical aspects of interactive systems.