When you let SW create a mirror part in an assy, and actually create the new mirrored part, it reproduces the original pretty well, including multiple configurations. If you modify the original part geometry, the mirrored part updates right along with it. But if you add a config to the original part, it does not show up in the mirrored part, as far as I can see. The only solution I have found is to remirror-create it, which gives the new file another name, close anything referencing the parts, delete the original mirror part, rename the new mirror part to the old name, and then reopen the assy's. That way any mates should still work.
Anything I have missed here?