Intel's Fab-18 is based in Israel, and has transitioned from producing 0.18-micron logic devices to producing 90nM flash products. During this transition period, the factory has de-ramped in volume of logic while ramping-up flash. AutoSched AP software was utilized for the development of a transient simulation model of the Fab's behavior during this period. It is the first attempt, at Intel, to utilize a full factory simulation in order to analyze and support decisions that pertain to a transient period of parallel de-ramp and ramp-up of technologies. Unlike typical simulation models for the analysis of factory performance and behavior in steady-state, the transient model poses several modeling challenges and requires major adjustments in dealing with these challenges. In this paper, we discuss those aspects. The benefits and contribution of such a model to decision making and the improvement of factory performance are also presented.