How to handle tests for changed requirements, or requirement change process overall

How to handle test cases for a requirement? 

Example: I have created a test case linked to a requirement, the test case is executed then the requirement changes. There seems to be no way to trigger an event for this change? How do I know which tests to re-test? Can I find the associated tests for a changed requirement by filter/search somehow? If not, is there any other recommended way to handle requirement change? I can't seem to find a way to handle requirement versions, e.g. v1 of the test case is executed towards v2 of the requirement? What is best practice in SpiraTest for this?