Ontologies are shared conceptualizations of certain domains. Especially in legal and regulatory ontologies modifications like the passing of a new law, decisions by high courts, new insights by scholars, etc. have to be considered. Otherwise, we would not be able to identify which knowledge (which ontology) was valid at an arbitrary timepoint in the past. And without this knowledge we would for instance not be able to identify why a user came to a specific decision. In this paper we will show how a simple ontology description formalism, namely a directed graph, has to be extended to represent changing knowledge. Furthermore, we will present the operations that are necessary to manipulate such an ontology. Finally, we will discuss different implementation approaches.