In complex systems with long life cycles, it is expected that the requirements of these systems will change over time. Therefore, it is important to be able to upgrade the system accordingly over time. This is often accomplished by infusing new technologies into the host system in order to update or improve the overall system performance. However, technology infusion often requires a redesign of the host system, resulting in a change in the overall system architecture complexity. In this study, we used the structural complexity index to analyse the impact of technology infusion by performing three main steps: (1) measurement of structural complexity, (2) Comparative Analysis A to determine the impact of components and their interfaces in each system and (3) Comparative Analysis B to determine the impact of technology infusion. To demonstrate the proposed process, we analyse technology infusion concepts for a hydrogen-enhanced combustion engine. Results show that the structural complexity index can be used as an indicator of the impact of design changes. To manage complex systems, decision-makers should consider the impact of technology infusion on the upgraded system, and select a better solution with high benefits and lower costs.