A major challenge of dynamic reconfiguration is Quality of Service (QoS) assurance, which is meant to reduce application disruption to the minimum for the system's transformation. However, this problem has not been well studied. This paper investigates the problem for component-based software systems from three points of view. First, the whole spectrum of QoS characteristics is defined. Second, the logical and physical requirements for QoS characteristics are analyzed and solutions to achieve them are proposed. Third, prior work is classified by QoS characteristics and then realized by abstract reconfiguration strategies. On this basis, quantitative evaluation of the QoS assurance abilities of existing work and our own approach is conducted through three steps. First, a proof-of-concept prototype called the reconfigurable component model is implemented to support the representation and testing of the reconfiguration strategies. Second, a reconfiguration benchmark is proposed to expose the whole spectrum of QoS problems. Third, each reconfiguration strategy is tested against the benchmark and the testing results are evaluated. The most important conclusion from our investigation is that the classified QoS characteristics can be fully achieved under some acceptable constraints.