In this paper we discuss the notions of type conversion and enhancement within the object-oriented paradigm. Type conversion allows an object to change from the type to which it belongs to another one, while enhancement allows an object to dynamically acquire properties of new types. By means of examples we demonstrate the usefulness of these two mechanisms for improving the modelling power of an object-oriented model. Furthermore we describe the nature of constraints that may be naturally captured when these mechanisms are available. Finally we outline the integration of type conversion and enhancement into typed object-oriented systems.