A knowledge base is maintained by modifying its conceptual model and by using those modifications to specify changes to its implementation. The maintenance problem is to determine which parts of that model should be checked for correctness in response a change in the application. The maintenance problem is not computable for first-order knowledge bases. Two things in the conceptual model are joined by a maintenance link if a modification to one of them means that the other must be checked for correctness, and so possibly modified, if consistency of the model is to be preserved. In a unified conceptual model for first-order knowledge bases the data and knowledge are modelled formally in a uniform way. A characterisation is given of four different kinds of maintenance links in a unified conceptual model. Two of these four kinds of maintenance links can be removed by transforming the conceptual model. In this way the maintenance problem is simplified.