Firms frequently apply more than one information system for their processes and data exchange. This variety is often due to the existence of legacy systems and can cause large inefficiencies. At present, most consumer goods firms aim at replacing non-integrated information flows with electronic data interchange. In doing so, firms constantly increase their level of information systems (IS) integration. However, firms have to decide whether they replace non-integrated legacy systems incrementally or radically. The paper investigates this issue from a conceptual viewpoint that combines the information technology (IT) substitution perspective with the concept of levels of IS integration. An empirical case study investigates two alternative IT substitution approaches, i.e., an incremental and a complete IT substitution, in respect of their impacts on process efficiency improvement. The results indicate that the complete IT substitution achieves significantly larger efficiency gains due to the impact of IS integration levels.