Modern applications distributed across networks such as the Internet may need to evolve without compromising application availability. Object sys- tems are well suited for runtime upgrade, as encapsulation clearly separates in- ternal structure and external services. This paper considers a mechanism for dy- namic class upgrade, allowing class hierarchies to be upgraded in such a way that the existing objects of the upgraded class and of its subclasses gradually evolve at runtime. The mechanism is integrated in Creol, a high-level language which targets distributed applications by means of concurrent objects communicating by asynchronous method calls. The dynamic class construct is given a formal semantics in rewriting logic, extending the semantics of the Creol language.