Check out Stephen Deutch's Label Management Tool user macro.
As Davin said, the Label Management Tool should be able to help you out.
Are you running OnDemand/Confluence Cloud? If so, feel free to comment this answer; I have written a Confluence Connect Add-on that also allows you to rename labels (and also add/remove watchers and move/delete pages), and I'm looking for a few beta testers, so if you want to try it out, let me know.
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