In siteCatalyst, modifying a shared segment affects all dashboards/bookmarks/custom reports for all users who have used the segment.
At the moment there is no way to tell where share segments are being used (or even if they are being used). So we can't make changes without risking breaking stuff.
Please can you consider introducing some kind of audit capability that will allow us to understand which users are using which shared segments in which reports?
Great suggestion, Andrew. Thanks for submitting.