Depends on your process, I've mostly updated the current projects test matrix to add any new items for regression purporses, then reuse it for the next one adding new cases to a different section. We use the test plan more as high level to capture what we will test and how, the specifics of the cases we keep in a matrix and that is constantly updated as we find new issues.
Nothing learns better than experience.
"So as I struggle with this issue I am confronted with the reality that noting is perfect."
Was it a bug or a feature request?
Cos I don't think a bug would have impact on high level documents. I think a bug is something that violates the specs.
Anyway if it is a feature request, in my company we would release a hotfix or patch with limited document updates, according to the process.
Thats right, mostly processes around these situation varies in every organization, but mostly, existing documents (test cases, plans ) are used to create a smaller version which is specific to that particular release.