It looks like there is a recent problem with iTunes Connect which prevents editing (changing any detail) of existing leaderboards.
A few days ago iTunes Connect had an issue where we could not create a new leaderboard. As of last night a new deployment of iTunes Connect fixed that particular bug, although it seems like a new issue was introduced.
The new issue is now that an existing leaderboard can no longer be edited, failing with an error upon save. The text of the error suggests the changes made to the leaderboard in the iTunes Connect associated web page are in fact resubmitted to the server as a new leaderboard, instead of an update to an existing entry.
The error is 100% reproducible and says:
The following error(s) occurred: - A leaderboard with ID " [leaderboard id] " already exists - The Leaderboard Reference Name you entered has already been used. Enter a unique Reference Name and try again."
In my case, the leaderboard was part of a leaderboard set.
I have submitted a bug report to Apple and will update when I know more. unless anyone finds a workaround or more details.
Apple have finally fixed the issue and the leaderboards are editable again. Thanks everyone.