Our company is using Rally to manage the test cases for each user story (we are using Agile process).
With the new user stories be added, some finished user stories become obsolete with the test cases (because of new flow and changed UI).
Then I have a problem of how to manage the obsolete test cases since these cases are not valid any more.

Currently, what I can think of are:
1. For the few impact on the finished user story due to new requirement (new user story added):
If there is only a few impact on the finished user stories and they are still valid, then change the old test cases due to requirement change.
2. For the huge impact on the finished user story (not valid any more)
1) attach the old test cases to the new user story and change according to the new requirement.
2) Just leave the old test cases there and create new test cases for the new user story

If you are in agile process and met this problem before, can you share your experience? Thanks a lot.