Ohlsson's proposal of resubsumption as the dominant process in conceptual, or nonmonotonic, change presents a worthy challenge to more established theories, such as Chi's theory of ontological shift. The two approaches differ primarily in that Ohlsson's theory emphasizes a process of learning in which narrower, more specific concepts are subsumed by more general, abstract categories through recognition of similarities, whereas Chi's theory emphasizes the dissociation of overly general categories through the recognition of differences. We examine the evidence for both theories and consider the educational implications of each. Overall, though subsumption almost certainly plays a role in nonmonotonic change, we maintain, on the basis of evidence from cognitive science and developmental psychology, that dissociation accounts for a greater portion of the re-representational processes underpinning changes in the structure of learner's knowledge.