Updating a current test plan after adding and deleting test cases

At the moment when you want to add/delete test cases from a current executing test plan, you have to add/delete test cases to/from test plan then create a new test cycle with the same software.  All the previous results from the previous test cycle will be lost.

Suggestion:
A way to update the test plan after adding/deleting test cases.  If it is the same current build, then carry over the previous results.