Requirement linked to test case must not change
Accepted answer
This scenario is one that we address elegantly in the 6.0 beta milestones. In V6 the RM and QM applications both have native configuration management support, and global configurations provide the context for the links between artifacts in both tools. To learn more, you can look in the jazz.net blog for some pointers.
But those capabilities are not in a production-ready release at the moment, and not every team will want to enable configuration management given the additional complexity and other trade-offs. So as a simpler solution that people use now ... if you want to keep a test case link to a particular version of a requirement and yet continue to evolve the requirement, you can copy the requirement and modify one of the two copies. This creates issues: you get a different artifact ID, and you have to manually deal with creating/removing links when you want the test case to point to a later version.