Regression Testing.
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We've planned to have a master plan which highlights the project test strategy and then each child test plan links to a test phase. Are you saying you would assign the phase in the TCERs
Yes - why use multiple test plans when you are really just going through milestones or iteration of the test. That is exactly what the test schedule is used for - you can divide up your test anyway you choose, for example you could have iterations(milestones) of Functional Verification Test (FVT), System Verification Test(SVT) and finally Regression Test. Each of these milestones can have its own duration and you can assign TCERs to each of the milestones (you can have finer grain milestones inside of these - we use weekly milestones and have them assigned to the appropriate milestone - for example weeks 1-4 might be FVT and 5-12 might be SVT and 13-14 might be Regression. I think child test plans for each milestone makes it more complicated to manage and to role up status of how you are doing.
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I understand the point you mentioned. I agree with you, casting a Test case to Regression would be confusing for it's reusability.
I think what David mentioned above is the right approach.
Even if you still consider a separate Test plan for regression. You then don't need any thing else do to. All testcases added under it are for regression. However if regression testing if part of the overall testing effort for a release I would suggest to use what David has mentioned above. identified the Test phase when regression testing to be done. Generate TCER for Regression test phase, only for the test cases which needs to be regress tested.