Future middleware platforms will need to be more configurable in order to meet the demands of a wide variety of application domains. Furthermore, we believe that such platforms will also need to be reconfigurable, for example to enable systems to adapt to changes in the underlying systems infrastructure. A number of technologies are emerging to support this level of configurability and re-configurability, most notably middleware platforms based on the concepts of open implementation and reflection. One problem with this general approach is that widespread changes can often be made to the middleware platform, potentially jeopardizing the integrity of the overall system. This paper discusses the role of software architecture in maintaining the overall integrity of the system in such an environment. More specifically, the paper discusses extensions to the Aster framework to support the re-configuration of a reflective (component-based) middleware platform in a constrained manner. The approach is based on i) the formal specification of a range of possible component configurations, ii) the systematic selection of configurations based on a given set of non-functional properties, and iii) the orderly re-configuration between configurations, again based on formally specified rules.