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On the edit posts screen, click the label to select all posts with it applied and add the new label to all of them (Label actions -> New label). Then just delete the label.
You can try Google Bookmark extension. It allows you do create nested labels and see them in a tree.
There is no way to undelete issue labels. Once gone, it's gone. If you had exported a backup of the issues before removing the label(s) then you might be able to go back and reference that to find out which issues had what labels.
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