Programming languages provide various mechanisms to support information hiding. One problem with information hiding, however, is that providing a stable interface behind which to hide implementation details involves fixing in advance the services offered through the interface. We introduce a flexible approach to define and manage interfaces to achieve separation of concerns in evolving software. Our approach involves explicitly specifying interface and implementation classes for individual concerns, and automatically classifying implementation classes based on their relation to the interface. Our approach is supported by JMantlet, a tool that provides advanced interface management within an integrated development environment. We report on a case study of a large system that provides evidence that flexible interface management is desirable and adequately supported by our approach.