Most contemporary fourth-generation languages (4GL) are tightly coupled with the database, and other subsystems, that are provided by the vendor. As a result, organizations that wish to change database vendors are typically forced to rewrite their applications using the new vendor 4GL. The anticipated cost of this redevelopment can deter an organization from changing vendors, hence denying it the benefits that would otherwise result, e.g., the exploitation of more sophisticated database technology. If software re-engineering 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 several approaches for migrating 4GL-based information system applications. These approaches were developed as part of an industry-sponsored research project to construct a real-world software re-engineering toolset. Semantic issues identified during the preliminary design of the toolset are addressed. These issues are instantiated using examples from the project. The application of knowledge-based software re-engineering technology, which facilitates the implementation of the toolset, is also described.