The recent breakthroughs in microprocessor technologies coupled with advances in software engineering and communication techniques has resulted in significant growth of web-based computing in order to meet the growing needs and demands. Web-based techniques have transformed the World Wide Web (WWW) into a giant computing infrastructure which has been employed for modeling and simulation. The simulation models are usually developed using component based techniques, wherein the models are composed using components developed by the modeler and other third party manufacturers, that considerably reduce model development time. In such simulations, the components constituting the simulation models can be dynamically substituted to enable effective "What-if" analyses. Dynamic component substitution (DCS) also enables the levels of abstraction of the different sub-models to be reconfigured during simulation thereby providing a tradeoff between simulation overheads and model details. DCS may also be used to achieve fault-tolerance and enable simulation of multiple futures. This paper presents the issues involved in the design and implementation of DCS in WESE: A Web-based Environment for Systems Engineering.