Everyone can agree that it's helpful to have many automated tests (system tests and unit tests) and to have written test plans. But how do you keep those tests synchronized with your Test Case Management tool or with your written test plans?
In particular, how do you avoid the awkwardness of needing to update tests in both places?
I'm not aware of any silver bullet solution for this. To my knowledge there is no round-trip testing system. I think it will be necessary to update your test plan as you implement new tests.
Another option is to leave the test plan a little less specific and use it primarily as a starting point. Write the test plan and have it reviewed by the relevant persons. After this, ensure that everything in the plan is written and don't bother updating it with the extra things you write. The point of the test plan is to ensure you don't miss anything. There is less value in using it later to see what was tested.
If you are in an industry that must document everything, I think you'll have to do the work in both places, but for most purposes, this isn't necessary.