When Editing a Global Calculated Metric allow the title to be renamed.  Currently, if you rename a globally calculated metric, a new one is created.  You don't want to delete the old one because if you do, saved reports and dashboards will be affected.

If an admin is in the Report Suites > Edit Settings > General > Calculated Metrics > Under Manage, if X (delete) is selected, a warning should appear that says something like - any reports and/or dashboards that are using this metric will be affected, are you sure you want to delete? --  Currently, ...