In theory this may tie to JRA-1602 and JRA-3821, but I felt it was different based on the details of the other two items. I've run into the problem of wanting to consolidate two issue Types into a single one (New Feature & Improvement into Enhancement). Problem is I can't delete New Feature and Improvement because they are in use. I don't want those two values to show up in any future projects as a valid selection.
Perhaps a workable middle ground it to provide the ability to deactivate a type so that records which already have that type can remain (legacy data) but they would no longer be displayed in the Type list when creating new records.