Systems evolve with societal, business and technological changes. Because of these changes, socio-technical systems need to adapt to new situations that were unknown at the time of design. Good knowledge of system evolution can help with that adaption. Although the evolution of software and interactive systems has been broadly debated, little research has been conducted on the specific genre of systems and even less empirical research on the evolution of interactive software has been performed. We propose a three-dimensional framework which consists of what changes during the evolution of training simulators, what are the drivers for those changes and how the changes effect innovation and robustness of the training simulators. By reviewing the literature on training simulators, we argue for this framework. The contribution of the paper is a framework that can be used to carry out empirical studies on evolution of training simulators.