When you choose File > Save As in 10.8.2, a Save dialog appears. This has always been the case; I stress that point, because I want to impress upon you that Apple’s solution involves no new dialogs of any kind. The change is within that Save dialog. If the current document contains unsaved changes, the Save dialog now offers a checkbox, “Keep changes in original document.” Moreover, that checkbox is unchecked by default, meaning that if you do nothing, you will not keep the changes in the original document — thus restoring the expected behavior of Save As. And this checkbox can appear regardless of your settings in the document management checkboxes of the General preference pane.
This is much better than in OS X 10.8.0.