TestLink: Binding new version of TestCase with Requirement - Obsolete TestCase

I have problem with binding new version of test cases with requierments in Testlink.

1) I have binded TestCases (version 1) with Requeirments in TestLink

TAB: Requirement Specifications image

2) Test cases have been executed.

3) Application has been changed -> TestCases have been changed (New versions have been created).

Now it is impossible to bind new version of test case to previous requirement - because the test case has already been binded (first version of test case). It is also possible to delete previous version of test case - because it has been executed. (the button responsible for removing the link disappeared for this test case - red circle on the screen shot).

Which is good practice? How to handle this problem ?

Thank you in advance for advices.