Collaborations (between objects) are increasingly being recognized as fundamental building blocks to structure object-oriented design, and they have made their way into UML. But very often the first class aspect of a design level collaboration is lost during the detailed design process, making it difficult to keep good traceability between the design and the implementation. The problem is not simple, because for any given collaboration abstraction, there might be several possible design solutions depending on the many non-functional forces impacting a given application. We propose a process and an architecture in which the notion of collaboration is preserved from analysis to design and implementation, while allowing the designer to change his mind about which particular design trade-off is selected in order to face changing non-functional requirements during maintenance. We illustrate our approach with a case study inspired by the real example of a large French railway company attempting to adapt a flight reservation system to its own context.