We have a case open to consider this feature for a future release. Please up-vote this *question* (not this answer) to show your support for adding this feature.

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This is not changeable in the UI in FogBugz. For now, the only option is to remove the relevant rows from the `BugRelation` table directly in the database. This [requires direct access][1] (i.e. locally installed FogBugz). Whenever you make changes directly in the database, make sure to **back it up** first.

Note: adding and removing subcases to a case is possible. This answer concerns the **see also** links created when you type "case XYZ" into a case.


  [1]: http://fogbugz.stackexchange.com/questions/1126/can-i-run-direct-sql-queries-against-my-fogbugz-on-demand-database