Most contemporary fourth-generation languages (4GL) are tightly coupled with the database, and other subsystems provided by the vendor. As a result, organisations that wish to change database vendors are typically forced to rewrite their applications using the new vendor's 4GL. The anticipated cost of this redevelopment can deter an organisation from changing vendors, hence denying it the benefits that would otherwise result, e.g., the exploitation of more sophisticated database technology. If tools existed that could reduce the rewriting effort, the large upfront cost of migrating the organisation's applications would also be reduced, which could make the shift economically feasible. This paper describes two approaches for migrating 4GL-based information system applications and how these approaches are enabled using knowledge-based software engineering (KBSE) technology. Semantic issues identified during the design of the toolset are addressed. These issues are instantiated using examples from the project. The approaches described in this paper were developed as part of an industry-sponsored research project to construct a "real world" software re-engineering toolset. The project represents the first significant attempt to apply KBSE technology to this problem.