If I select a corner component on one master and Cmd-C copy it, then switch to a different master, select the corresponding corner component, and Cmd-V paste it, the component name changes as if the pasting replaced the old component with the new one. But the component itself doesn’t change, and switching away from and back to that master makes the label go back to the old one.
I’d expect the paste-to-replace-selection to work the same way for corners and caps as it does for contours. Or at any rate if the replacement won’t work the feedback about that should be clearer.
Edited to add: actually looks like the component is pasted “below” the old one–when I delete the old one the new one shows up.